Amino resin crosslinked particles and producing process thereof

ABSTRACT

A producing process of amino resin crosslinked particles includes the step of curing an emulsion of an amino resin precursor and an emulsifier in the presence of a catalyst, the amino resin precursor being a product of a reaction of an amino compound with formaldehyde, so as to prepare amino resin particles, the step of neutralizing the suspension of the amino resin particles after the curing step, and the step of heating the suspension in a temperature range of 130° C. to 230° C. after the neutralizing step. The amino resin crosslinked particles thus prepared by the condensation of the amino compound and formaldehyde generate formaldehyde in an amount of not more than 1000 ppm in a pyrolysis test and have a characteristic that a Hunter whiteness is not less than 85 percent.

FIELD OF THE INVENTION

The present invention relates to amino resin crosslinked particles whichcan be suitably used, for example, in flatting agents; light diffusingagents; polishing agents; coating agents for various films; fillers thatare added to polyolefin, polyvinyl chloride, various types of rubbers,paints, toners, and the like; rheology control agents; and coloringagents. The invention also relates to producing processes of such aminoresin crosslinked particles.

BACKGROUND OF THE INVENTION

Conventional processes for producing amino resin crosslinked particlesare disclosed, for example, in Japanese Publication for UnexaminedPatent Application Nos. 57091/1974 (Tokukaisho 49-57091) (published onJun. 3, 1974), 45852/1975 (Tokukaisho 50-45852) (published on Apr. 24,1975), and 211450/1992 (Tokukaihei 4-211450) (published on Aug. 3,1992). The producing processes of amino resin crosslinked particles astaught by these publications proceed as follows. First, an aminocompound is allowed to react with formaldehyde to prepare an amino resinprecursor. The amino resin precursor is then mixed with an aqueoussolution of an emulsifier to obtain an emulsion. Thereafter, a catalystis added to the emulsion to cure the amino resin precursor in theemulsion state and obtain a suspension of amino resin particles.Subsequently, the amino resin particles are separated from thesuspension and the cake containing the resulting amino resin is heatedand dried to remove water from the cake and to complete cure(condensation) of the amino resin particles. One of the problems of theproducing processes of the foregoing publications is that the aminoresin crosslinked particles discolor. For example, the amino resincrosslinked particles may be discolored to yellow or other colors notoriginally intended. In the following explanations, the term“discoloring” will be used to indicate such a phenomenon in which theamino resin crosslinked particles are discolored to yellow or othercolors not originally intended.

The amino resin crosslinked particles discolor because the amino resinparticles are subjected to high temperatures when the cake is heated ata temperature above 100° C., for example, in a temperature range of 130°C. to 230° C., in a heating process that is provided to remove themoisture and unreacted formaldehyde contained in the cake of amino resinparticles separated from the suspension, so as to improve the efficiencyof condensation of the amino resin particles. It is therefore requiredto set a low heating temperature of, for example, not higher than 100°C. to prevent discoloring of the amino resin crosslinked particles.

A drawback of low temperature heating (e.g., 100° C. or below) inheating the amino resin particles is that heating takes an extendedperiod of time and removal of the moisture and free formaldehyde becomesinsufficient.

Another drawback is that condensation (crosslinking) of the amino resinparticles becomes insufficient, with the result that hardness, heatresistance, and solvent resistance of the product amino resincrosslinked particles degrade.

It is therefore required, in order to improve hardness, heat resistance,and solvent resistance of the amino resin crosslinked particles, to heatthe amino resin particles at a temperature above 100° C. (for example,130° C. to 230° C.). Heating the amino resin particles at such atemperature causes the problem of discoloring on the amino resincrosslinked particles.

As the catalyst (curing catalyst) used to cure the amino resinprecursor, various types of acid catalysts have been usedconventionally. For example, the foregoing Tokukaisho 49-57091 andTokukaisho 50-45852 use dodecylbenzenesulfonic acid (“DBS” hereinafter)as the catalyst. One drawback of DBS is its relatively slow reactionspeed, which requires higher temperature heating or a process of longtime heating. Another drawback of DBS is attributed to its color, brown,which causes the color of the amino resin particles to change in theheating process of the amino resin particles. In other cases, the DBSused as the catalyst may permeate into the amino resin particles tocause plasticization and to prevent stable crosslinking.

Further, Japanese Publication for Unexamined Patent Application No.72015/1981 (Tokukaisho 56-72015) (published on Jun. 16, 1981) usessulfuric acid as the catalyst. The sulfuric acid has no color and doesnot slow the reaction speed. However, specific examples of amino resincompositions described in this publication contain paratoluenesulfonamide (PTSA) as the amino compound. Containing PTSA as the aminocompound is disadvantageous in the following respect. Namely, itdiscourages condensation because of small numbers of crosslinking sitespresent between the amino compound and formaldehyde. The product aminoresin crosslinked particles, as a result, have insufficient hardness andinsufficient heat resistance. That is, paratoluene sulfonamide is notpreferable as the amino compound where high levels of hardness and heatresistance are required for the amino resin crosslinked particles.Further, the foregoing Japanese koukai publication does not describeneutralization of acid catalyst. The conventional techniques thereforefail to produce amino resin crosslinked particles with sufficienthardness and heat resistance, and without causing discoloring uponheating.

SUMMARY OF THE INVENTION

An object of the present invention is to provide amino resin crosslinkedparticles which are superior in hardness, heat resistance, and solventresistance, and which do not discolor. It is also an object of thepresent invention to provide producing processes of such amino resincrosslinked particles.

As the term is used herein, “amino resin particles” indicate thoseparticles that exist as a cured emulsion of an amino resin precursor, orthose particles that exist in a reaction solution in a producing step.Also, the term “amino resin crosslinked particles” will be used to referto particles that are obtained as the final product, by separating theamino resin particles from a suspension and by carrying out a series ofproducing steps, such as a drying step.

After extensive research on amino resin crosslinked particles andproducing processes thereof to achieve the foregoing objects, theinventors of the present invention have found that the objects can beachieved by a process that proceeds as follows. A catalyst is added toan emulsion that contains the amino resin precursor, which is obtainedby a reaction of an amino compound with formaldehyde, and an emulsifier,so as to cure the amino resin precursor in an emulsion state and preparea suspension of amino resin particles. A suspension of the amino resinparticles is then adjusted within a specific pH range. The amino resinparticles are finally separated from the emulsion and heated within aspecific temperature range.

That is, the present invention was made based upon the finding thatamino resin crosslinked particles that do not discolor under hightemperature heating after curing can be obtained when an acid catalystused for curing is neutralized (removed) by adjusting a suspension ofthe amino resin particles within a specific pH range before heating theamino resin particles, the suspension of the amino resin particles beingobtained by curing the amino resin precursor in an emulsion state. Theamino resin crosslinked particles had a structure to sufficiently resistheat and had a unique property not to discolor in a heating step, suchas a drying step after curing or after production. Another property ofthe amino resin crosslinked particles was that the amount of residualformalin was small, owning to the fact that the amino resin crosslinkedparticles are sufficiently treated with heat. The amino resincrosslinked particles also had a structure that resists pyrolysis, whichrendered the amino resin crosslinked particles a property of generatingonly a small amount of formaldehyde in a pyrolysis test. That is, thepresent invention is based upon a finding that a main reason fordiscoloring of the amino resin particles in a heating step after curingis the presence of a remaining acid catalyst that is used as a curingcatalyst. The present invention, based upon this finding, solves theproblem of discoloring by neutralizing the acid catalyst.

The amino resin crosslinked particles of the present invention, producedthis way, is the product of condensation of an amino compound withformaldehyde, the amino resin crosslinked particles having acharacteristics that an area ratio of a carbon atom signal thatoriginates from an —NH—CH₂—NH— bond to a carbon atom signal thatoriginates from an —NH—CH₂O—CH₂—NH— bond in a solid-state ¹³C-NMRanalysis is not less than 2, and the amino resin crosslinked particleshaving a characteristic that a Hunter whiteness is not less than 85percent.

Further, the amino resin crosslinked particles of the present invention,which are obtained by condensation of an amino compound withformaldehyde, generate formaldehyde in an amount of not more than 1000ppm in a pyrolysis test, and have a characteristic that a Hunterwhiteness is not less than 85 percent.

The amino resin crosslinked particles of the present invention also havea triazine ring and are formed by condensation of formaldehyde with anamino compound having a triazine ring, wherein the amino resincrosslinked particles have a characteristic that an area ratio of acarbon atom signal that originates from an —NH—CH₂—NH— bond to a carbonatom signal that originates from the triazine ring of the amino resincrosslinked particles in a solid-state ¹³C-NMR analysis is not less than0.20.

Further, the amino resin crosslinked particles of the present inventionpreferably contains at least one kind of compound, in a range of 40percent by weight to 100 percent by weight, selected from the groupconsisting of benzoguanamine, cyclohexanecarboguanamine,cyclohexenecarboguanamine, and melamine, and a mole ratio of the aminocompound to the formaldehyde is preferably in a range of 1:1.5 to 1:3.5.

In the configuration where a Hunter whiteness is not less than 85percent and where an area ratio of a carbon atom signal that originatesfrom an —NH—CH₂—NH— bond (C(II) bond) to a carbon atom signal thatoriginates from an —NH—CH₂O—CH₂—NH— bond (C(I) bond) in a solid-state¹³C-NMR analysis is not less than 2, the proportion of the—NH—CH₂O—CH₂—NH— bond is relatively small. With this, the amino resincrosslinked particles generate only a small amount of formaldehyde whenheated. Further, because of the large number of —NH—CH₂—NH— bonds makingup the amino resin crosslinked particles, the product amino resincrosslinked particles are superior in hardness, heat resistance, andsolvent resistance.

The proportion of —NH—CH₂O—CH₂—NH— bonds in amino resin crosslinkedparticles becomes large when the curing and condensation areinsufficient because of an insufficient heat treatment. Amino resincrosslinked particles containing a large number of —NH—CH₂O—CH₂—NH—bonds easily generate formaldehyde when heated. That is, the area ratioof a carbon atom signal that originates from an —NH—CH₂—NH— bond to acarbon atom signal that originates from an —NH—CH₂O—CH₂—NH— bond is ameasure of how small the amount of formaldehyde generated from the aminoresin crosslinked particles is in the pyrolysis test and how superiorthe hardness, heat resistance, and solvent resistance of the amino resincrosslinked particles are. Another superior property of the amino resincrosslinked particles of the present invention is that the particleshardly discolor.

The acid catalyst is also used in the curing step when the amino resincrosslinked particles of the present invention are colored with a dyeand/or a pigment. Hence, discoloring upon heating caused by a remainingacid catalyst can be limited in the same manner for colored amino resincrosslinked particles by the process of neutralizing the acid catalyst.Thus, in the case of colored amino resin crosslinked particles, a degreeof discoloring upon heating is evaluated using a measure of “colordifference” in a heat discoloring test. In this case, in order toachieve the foregoing objects, colored amino resin crosslinkedparticles, which are the product of condensation of an amino compoundwith formaldehyde, have a characteristic that an area ratio of a carbonatom signal that originates from an —NH—CH₂—NH— bond to a carbon atomsignal that originates from an —NH—CH₂O—CH₂—NH— bond in a solid-state¹³C-NMR analysis is not less than 2, and have a characteristic that acolor difference is not more than 15 in a heat discoloring test.

Further, the colored amino resin crosslinked particles of the presentinvention, which are obtained by condensation of an amino compound withformaldehyde, generate formaldehyde in an amount of not more than 1000ppm in a pyrolysis test, and have a characteristic that a colordifference is not more than 15 in a heat discoloring test.

The colored amino resin crosslinked particles of the present inventionalso have a triazine ring and are formed by condensation of formaldehydewith an amino compound having a triazine ring, wherein the colored aminoresin crosslinked particles have a characteristic that an area ratio ofa carbon atom signal that originates from an —NH—CH₂—NH— bond to acarbon atom signal that originates from the triazine ring of the coloredamino resin crosslinked particles in a solid-state ¹³C-NMR analysis isnot less than 0.20, and wherein the colored amino resin crosslinkedparticles have a characteristic that a color difference is not more than15 in a heat discoloring test.

It is preferable in the colored amino resin crosslinked particles of thepresent invention that the amino compound contains at least one kind ofcompound, in a range of 40 percent by weight to 100 percent by weight,selected from the group consisting of benzoguanamine,cyclohexanecarboguanamine, cyclohexenecarboguanamine, and melamine, anda mole ratio of the amino compound to the formaldehyde is in a range of1:1.5 to 1:3.5.

With this configuration, the colored amino resin crosslinked particleshave a characteristic that a color difference is not more than 15 in aheat discoloring test and contain a relatively small proportion of the—NH—CH₂O—CH₂—NH— bond, owning to the fact that an area ratio of a carbonatom signal that originates from an —NH—CH₂—NH— bond to a carbon atomsignal that originates from an —NH—CH₂O—CH₂—NH— bond in a solid-state¹³C-NMR analysis is not less than 2. Thus, as with the colorless aminoresin crosslinked particles, the product colored amino resin crosslinkedparticles generate only a small amount of formaldehyde upon heating.Further, because of the large number of —NH—CH₂—NH— bonds making up thecolored amino resin crosslinked particles, the product amino resincrosslinked particles are superior in hardness, heat resistance, andsolvent resistance.

A producing process of amino resin crosslinked particles of the presentinvention includes the steps of (1) adding a catalyst to an emulsion ofan amino resin precursor that is obtained by mixing a reaction solutioncontaining the amino resin precursor, which is obtained by allowing anamino compound to react with formaldehyde, with an aqueous solution ofan emulsifier and/or a surfactant, so as to cure the amino resinprecursor in an emulsion state and obtain a suspension of amino resinparticles, (2) neutralizing the suspension of the amino resin particlesobtained in step (1), and (3) separating the amino resin particles fromthe suspension after step (2) and heating the amino resin particles in atemperature range of 130° C. to 230° C. With such a process includingthe neutralizing step for neutralizing the acid catalyst, the foregoingproperties are rendered to the amino resin crosslinked particles and thecolored amino resin particles.

The neutralizing step adjusts the suspension that contains the aminoresin particles from a pH of 1.5 to 3 to a pH of not less than 5. Theheating step is preferably carried out in an atmosphere of inert gasthat contains oxygen in a concentration of not more than 10 percent byvolume.

It is preferable in the producing process of amino resin crosslinkedparticles of the present invention that the step of (1) generating aminoresin particles comprises a step of coloring the amino resin precursorwith a dye and/or a pigment.

It is also preferable in the producing process of amino resincrosslinked particles of the present invention that the dye is afluorescent dye and the pigment is a fluorescent pigment.

With the producing process of amino resin crosslinked particles of thepresent invention, by neutralizing the suspension that contains theamino resin particles after the curing step and after the step of (1)generating amino resin particles, a remaining acid catalyst can beremoved. It is therefore possible to suppress discoloring of the aminoresin particles in the subsequent heating step. More specifically,heating the amino resin particles in a temperature range of 130° C. to230° C. enables water and free formaldehyde to be removed and promotescondensation. As a result, the amino resin crosslinked particles do notdiscolor and are superior in hardness, heat resistance, and solventresistance, and generate only a trace amount of formaldehyde in thepyrolysis test.

In addition, by carrying out the heating step in an atmosphere of inertgas that contains oxygen in a concentration of not more than 10 percentby volume, discoloring of the amino resin crosslinked particles can beprevented even more effectively.

Further, in order to achieve the foregoing objects, the amino resincrosslinked particles of the present invention are produced by a processthat includes the steps of adding a catalyst to an emulsion that isobtained by mixing an amino resin precursor, that is obtained byreacting an amino compound with formaldehyde, with an aqueous solutionof a surfactant, so as to cure the amino resin precursor and obtainamino resin particles, and heating the amino resin particles in anatmosphere of inert gas that contains oxygen in a concentration of notmore than 10 percent by volume and in a temperature range of 130° C. to230° C.

Note that, as disclosed herein, the producing process that includes thestep of neutralizing the acid catalyst used in the curing step in thestep of producing the amino resin particles is applicable to colorlessamino resin crosslinked particles, or more specifically white aminoresin crosslinked particles, as well as colored amino resin crosslinkedparticles. As the term is used herein, “white particles” are thoseparticles that contain no coloring agent such as a dye (including awhite dye) or a pigment (including a white pigment). Therefore, theFirst Embodiment of the present invention describes colorless or whiteamino resin crosslinked particles that contain no coloring agent. TheSecond Embodiment describes colored amino resin crosslinked particles.It should be noted that white amino resin crosslinked particles, thatare colored with a white pigment or a white dye, may be evaluated usinga Hunter whiteness, as described in the First Embodiment, or using acolor difference in a heat resistance test (heat discoloring test), asdescribed in the Second Embodiment.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a solid-state ¹³C-NMR analysis spectrum of amino resincrosslinked particles of Example 1.

FIG. 2 is a solid-state ¹³C-NMR analysis spectrum of amino resincrosslinked particles of Comparative Example 1.

FIG. 3 is a solid-state ¹³C-NMR analysis spectrum of amino resincrosslinked particles of Example 4.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

One embodiment of the present invention is described below. Amino resincrosslinked particles according to the present invention are produced bycondensation of an amino compound with formaldehyde, wherein an arearatio of a carbon atom signal that originates from an —NH—CH₂—NH— bondto a carbon atom signal that originates from an —NH—CH₂O—CH₂—NH— bond ina solid-state ¹³C-NMR analysis is not less than 2, and wherein a Hunterwhiteness is 85% or greater.

Specific examples of the amino compound used in the present inventioninclude benzoguanamine, cyclohexanecarboguanamine,cyclohexenecarboguanamine, and melamine. Of these compounds,benzoguanamine, having a benzene ring and two reactive groups, isparticularly preferable because it has ease of dyeing in the initialstage of condensation and it is superior in flexibility (hardness),stain resistance, heat resistance, solvent resistance, and chemicalresistance after the compound has crosslinked. These amino compounds maybe used individually or in a mixture of two or more kinds.

In either case, it is preferable that at least one kind of compoundselected from the foregoing group is used in an amount of not less than40 percent by weight and not more than 100 percent by weight. Further,the amino compound of the present invention preferably include atriazine ring. Benzoguanamine compounds are particularly preferable.

In the following, the descriptions of amino compounds based on chemicalformulae will be given through the case of benzoguanamine.

Structures of amino resin crosslinked particles are described below. Theamino resin crosslinked particles according to the present invention areobtained by condensation of the amino compound with formaldehyde.Heating the condensate (amino resin particles) of the amino compound andformaldehyde at a relatively low temperature (specifically, less than130° C.) in a heating process (described later) produces amino resincrosslinked particles with a large proportion of the —NH—CH₂O—CH₂—NH—bond (“C.(I) bond” hereinafter), which is created by the condensation of1 mole of amino compound and 2 moles of formaldehyde, in their repeatingstructure units, as shown in Formula (1) below.

Note that, Formula (1) only shows a state of condensation of the aminocompound and formaldehyde between benzoguanamine structures. That is,Formula (1) does not represent a repeating structure unit. Specifically,1 mole of the amino compound reacts with 2 moles of formaldehyde toyield an amino compound with methylol groups. By the dehydrocondensationof the methylol groups of the amino compound, the —NH—CH₂O—CH₂—NH— bond(C(I) bond) is formed between the amino groups of the amino compound.

Heating the condensate of the amino compound and formaldehyde at arelatively high temperature (specifically, 130° C. to 230° C.) in theheating process (described later) produces amino resin crosslinkedparticles which include a different type of bond, as indicated by C(II)in Formula (2) below.

That is, amino resin crosslinked particles with a large proportion of an—NH—CH₂—NH— bond (“C.(II) bond” hereinafter) are obtained. Note that,Formula (2) only shows a state of bonding between the compounds ofbenzoguanamine. That is, Formula (2) does not represent a repeatingstructure unit. Specifically, 1 mole of the amino compound reacts with 2moles of formaldehyde to yield an amino compound with methylol groups.By the dehydrocondensation of the methylol groups of the amino compound,the —NH—CH₂O—CH₂—NH— bond (C(I) bond) is formed between the amino groupsof the amino compounds. Eliminating formalin forms the —NH—CH₂—NH— bond(C(II) bond).

The proportions of the C(I) bond and the C(II) bond in the amino resincrosslinked particles change in such a way that the proportion of theC(I) bond increases as the heating temperature is decreased (100° C. andbelow), and conversely the proportion of the C(II) bond increases as theheating temperature is increased (130° C. and above). Where the aminoresin crosslinked particles include more C(I) bonds than C(II) bonds,the amino resin crosslinked particles easily generate formaldehyde whenthey are heat-decomposed. In addition, heat resistance and solventresistance of the amino resin crosslinked particles become poor. On theother hand, where the amino resin crosslinked particles include onlyC(II) bonds, no formaldehyde generates even when the amino resincrosslinked particles are decomposed. The relations between theproportions of the C(I) and C(II) bonds and a heating temperature varydepending on such factors as the type of amino compound, reactionconditions, and reaction steps.

According to these findings, the inventors of the present inventionconducted in-depth investigation of relations between structuralfeatures of the product amino resin crosslinked particles and variousproperties of the particles, including heat resistance, solventresistance, and an amount of formaldehyde generated in a pyrolysis test,for example. One feature of the amino resin crosslinked particles thatwas particularly closely looked into was proportions of the C(I) bondand C(II) bond in the amino resin crosslinked particles. It was found bythe analysis of proportions of the C(I) bond and C(II) bond of the aminoresin crosslinked particles that the relations between the structuralfeatures of the amino resin crosslinked particles and their properties,such as hardness, heat resistance, solvent resistance, and an amount offormaldehyde they generate in a pyrolysis test can be clearly defined.The proportions of the C(I) bond and C(II) bond can therefore be used asa clear index that is indicative of properties of the amino resincrosslinked particles of the present invention.

It is preferable in the amino resin crosslinked particles according tothe present invention that the area ratio of a carbon atom signal thatoriginates from the C(II) bond to a carbon atom signal that originatesfrom the C(I) bond in a solid-state ¹³C-NMR analysis (NMR area ratioindicated by C(II)/C(I) in the embodiments) is not less than 2,preferably within a range of 2 to 20, or more preferably a range of 2 to10.

The area ratio indicates a state of bonding in the amino resincrosslinked particles, i.e., a state of bonding between the aminocompounds and formaldehyde making up the amino resin crosslinkedparticles. When the area ratio of a carbon atom signal that originatesfrom the C(II) bond to a carbon atom signal that originates from theC(I) bond in a solid-state ¹³C-NMR analysis is less than 2, the hardnessof the product amino resin crosslinked particles becomes weak and itdegrades heat resistance and solvent resistance of the product aminoresin crosslinked particles.

It is preferable in the amino resin crosslinked particles according tothe present invention that formaldehyde is generated in an amount of notmore than 1000 ppm, preferably not more than 500 ppm, more preferablynot more than 300 ppm, even more preferably not more than 100 ppm, andmost preferably not more than 50 ppm, in a measured value in a pyrolysistest. More desirably, the amount of formaldehyde generated should beless than the practical limit of detection by a detector, i.e., anundetectable amount by a detector. A generated amount of formaldehydeexceeding 1000 ppm in the pyrolysis test means the amino resincrosslinked particles have a large proportion of C(I) bonds. Thehardness of the product amino resin crosslinked particles becomes weakin this case, which may degrade heat resistance and solvent resistanceof the amino resin crosslinked particles. Note that, as the term is usedherein, “pyrolysis test” is a test that is used to measure the generatedamount of formaldehyde, by heating the amino resin crosslinked particlesat 160° C.

It is preferable in the amino resin crosslinked particles according tothe present invention that the area ratio in a solid-state ¹³C-NMRanalysis of a carbon atom signal that originates from the C(II) bond toa carbon atom signal that originates from the triazine ring (C(IV) bond)of the amino compound of the amino resin crosslinked particles (NMR arearatio indicated by C(II)/C(IV) in the embodiments) is not less than0.20, or more preferably within a range of 0.20 to 0.40. The carbonatoms that originate from the triazine ring are the three carbon atoms,as shown in Formula (2), that make up the triazine ring (“C(IV)”hereinafter). The area ratio of a carbon atom signal that originatesfrom the C(II) bond to a carbon atom signal that originates from theC(IV) bond becomes less than 0.20 when the number of C(II) bonds issmall. In this case, the proportion of the C(I) bond becomes large andthe hardness of the product amino resin crosslinked particles becomesweak, which may degrade heat resistance and solvent resistance of theproduct amino resin crosslinked particles.

It is preferable that the amino resin crosslinked particles according tothe present invention has a characteristic that a Hunter whiteness isnot less than 85%, or more preferably not less than 90%. “Hunterwhiteness” is a JIS standard under P8123 of the regulation (“Hunterwhiteness testing method for paper and pulp”), and it is a measure ofwhiteness with respect to a reference color of 100% white. Morespecifically, Hunter whiteness is a relative reflectance of the sampleirradiated with light with respect to a reference magnesium oxide board.A spectral calorimeter, for example, is used to measure Hunterwhiteness. A Hunter whiteness of less than 85% is not preferable becauseit discolors the amino resin crosslinked particles to yellow(“discoloring” hereinafter). Note that, in the present invention, when aHunter whiteness of the product amino resin crosslinked particles isless than 85%, the product amino resin crosslinked particles are deemedas the discolored particles.

As described, it is preferable in the amino resin crosslinked particlesaccording to the present invention that (i) the Hunter whiteness is notless than 85% and the area ratio of a carbon atom signal that originatesfrom the C(II) bond to a carbon atom signal that originates from theC(I) bond in a solid-state ¹³C-NMR analysis is not less than 2, (ii) theHunter whiteness is not less than 85% and the amount of formaldehydegenerated is not more than 1000 ppm in a measured value of a pyrolysistest, and (3) the Hunter whiteness is not less than 85% and the arearatio of a carbon atom signal that originates from the C(II) bond to acarbon atom signal that originates from the C(IV) bond in a solid-state¹³C-NMR analysis is not less than 0.20.

It is also preferable, when the amino compound is benzoguanamine asshown in Formula (3) below, that the area ratio in a solid-state ¹³C-NMRanalysis of a carbon atom signal that originates from the C(II) bond toa carbon atom signal that originates from a benzene ring (“C(III)”hereinafter) of the benzoguanamine structure (NMR area ratio indicatedby C(II)/C(III) in the embodiments) is not less than 0.08, preferablywithin a range of 0.08 to 0.20, or more preferably 0.10 to 0.20. Notethat, the C(II) bond is the —NH—CH₂—NH— bond.

The area ratio of a carbon atom signal that originates from the C(II)bond to a carbon atom signal that originates from the C(III) bondbecomes less than 0.08 when the C(I) bond and the C(II) bond coexist inthe amino resin crosslinked particles. In this case, the proportion ofthe C(I) bond becomes large and heat resistance and solvent resistanceof the product amino resin crosslinked particles may degrade.

It is preferable in the amino resin crosslinked particles according tothe present invention, which is obtained by condensation of an aminocompound with formaldehyde, that the mole ratio of structure units ofthe amino compound and formaldehyde making up the amino resincrosslinked particles is in a range of 1:1 to 1:2, and that the Hunterwhiteness is not less than 85%. The structure unit of the formaldehydemay be the methylene group (specifically, the C(II) bond), as indicatedin Formula (2), that results from the condensation of the amino compoundand formaldehyde, or the C(I) bond, as indicated in Formula (1), thatresults from the condensation of the amino compound and formaldehyde.The condensation may become insufficient when the mole ratio of thestructure units of the formaldehyde and the amino compound falls below 1(the structure units of formaldehyde/the structure units of the aminocompound). On the other hand, the proportion of the C(I) bond in theamino resin crosslinked particles becomes large when the mole ratio ofthe structure units of the amino compound and formaldehyde exceeds 2. Inthis case, the hardness of the product amino resin crosslinked particlesbecomes weak, which may lead to degradation of heat resistance andsolvent resistance of the amino resin crosslinked particles.

The following describes a producing method of amino resin crosslinkedparticles according to the present invention. A producing process ofamino resin crosslinked particles according to the present inventionincludes the steps of: (1) adding a catalyst to an emulsion that wasobtained by mixing an amino resin precursor, which is the product of areaction between an amino compound and formaldehyde, with an aqueoussolution of an emulsifier, so as to cure the amino resin precursor toprepare amino resin particles; (2) neutralizing a suspension thatcontains the amino resin particles obtained in step (1); and (3) heatingthe amino resin particles after step (2) in a temperature range of 130°C. to 230° C. Specifically, a catalyst is added to an emulsion of anamino resin precursor that is obtained by mixing an aqueous solution ofan emulsifier with a reaction solution containing the amino resinprecursor that results from the reaction of the amino compound withformaldehyde. The mixture is stirred to maintain an emulsion state andthe amino resin precursor is cured in the emulsion state to obtain asuspension that contains amino resin particles. Then, the suspensioncontaining the amino resin particles in a pH range of 1.5 to 3 isadjusted in the neutralizing step, so that the pH of the suspensionbecomes not less than 5, or more preferably 5 to 9. The amino resinparticles are then separated from the suspension and heated in. atemperature range of 130° C. to 230° C., preferably 130° C. to 210° C.,or more preferably 130° C. to 190° C.

The following explanations will be given through the case where the stepof preparing amino resin particles includes a reaction step, anemulsifying step, and a curing step.

In the reaction step, the amino compound is allowed to react withformaldehyde to obtain the amino resin precursor. The reaction of theamino compound with formaldehyde uses water as a solvent. Thus,formaldehyde may be added or charged in the form of an aqueous solution(formalin), or may be generated in a reaction solution by addingtrioxane or paraformaldehyde to water. Preferably, formaldehyde is addedin the form of an aqueous solution. The reaction step is carried out inthis manner to obtain a reaction solution containing the amino resinprecursor.

The mole ratio of the amino compound to formaldehyde is preferably in arange of 1:1.5 to 1:3.5, or more preferably 1:2 to 1:3.5. A proportionof formaldehyde outside these ranges is not preferable because itincreases an unreacted portion of the amino compound and formaldehyde.Note that, the amount of amino compound and formaldehyde added withrespect to water, i.e., the concentration of amino compound andformaldehyde when they are charged should be increased as high aspossible, provided that it does not hinder the reaction. Morepreferably, such a concentration is chosen that the viscosity of thereaction solution containing the reactant, the amino resin precursor, ina temperature range of 95° C. to 98° C. after the reaction falls withina range of 2×10⁻² Pa.s to 5.5×10⁻² Pa.s (20 cP to 55 cP) at 95° C. to98° C. More preferably, the concentration is such that addition of thereaction solution into the aqueous solution of the emulsifier yields 30percent by weight to 60 percent by weight of amino resin precursor inthe emulsifying step. That is, the amino resin precursor according tothe present invention is the initial condensate, which is obtained bythe reaction of the amino compound with formaldehyde, whose viscosity inthe reaction solution in a temperature range of 95° C. to 98° C. afterthe reaction is 2×10⁻² Pa.s to 5.5×10⁻² Pa.s (20 cP to 55 cP) at 95° C.to 98° C.

The viscosity can be measured most suitably with use of a viscometer, sothat proceedings of the reaction of the amino compound with formaldehydecan be grasped instantly (in real time) and an end point of the reactioncan be accurately found. An example of such a viscometer is thevibration viscometer (product of MIVI ITS JAPAN; Model No. MIVI 6001).This viscometer is equipped with a vibrator that is constantlyvibrating. When the vibrator dipped in the reaction solution experiencesa load in response to increased viscosity of the reaction solution, theviscometer instantly converts the load into a viscosity for display. Thevibration viscometer is also known as a process viscometer.

It is preferable that the pH of the reaction solution used to producethe amino resin precursor be adjusted to be either neutral or weaklybasic, using alkaline compounds such as sodium carbonate, sodiumhydroxide, potassium hydroxide, or ammonium water. The amount of thealkaline compound used should be adjusted by measuring it using a pHmeter or the like, so that the pH of the emulsion containing the aminoresin precursor falls in a predetermined pH range. The reaction of theamino compound with formaldehyde in water yields the amino resinprecursor, which is the initial condensate. The reaction temperatureshould preferably be in a range of 90° C. to 98° C., or more preferably95° C. to 98° C., so that the reaction of the amino compound withformaldehyde proceeds efficiently. The reaction step is ended in thetemperature range of 95° C. to 98° C., when the viscosity of thereaction solution becomes 2×10⁻² Pa.s to 5.5×10⁻² Pa.s in thetemperature range of 95° C. to 98° C., by cooling the reaction solution,for example. The product of the reaction step is a reaction solutioncontaining the amino resin precursor. Reaction time is not particularlylimited accordingly. It is preferable that the mole ratio of thestructure units of the amino compound and formaldehyde, which make upthe amino resin precursor obtained in the reaction step, is within arange of 1:1.5 to 1:3.5. With a mole ratio in this range, particles witha narrow particle distribution can be obtained.

Note that, the viscosity of the reaction solution at the end of thereaction is considerably higher than that of the aqueous solution (atthe start of the reaction) charged with the amino compound andformaldehyde, and therefore the viscosity of the reaction solution isessentially unaffected by the concentration or other variables of theraw materials. The amino resin precursor is practically insoluble inwater but soluble in organic solvents, examples of which includeacetone, dioxane, methyl alcohol, ethyl alcohol, isopropyl alcohol,butyl alcohol, ethyl acetate, butyl acetate, methyl cellosolve, ethylcellosolve, methylethyl ketone, toluene, and xylene.

The lower the viscosity of the reaction solution of the amino resinprecursor, the smaller the particle size of the particles in theemulsion step. However, amino resin crosslinked particles of essentiallyuniform particle size (narrow particle distribution) cannot be obtainedwhen the viscosity of the reaction solution is less than 2×10⁻² Pa.s orexceeds 5.5×10⁻² Pa.s in the temperature range of 95° C. to 98° C.Specifically, when the viscosity of the reaction solution is below2×10⁻² Pa.s (20 cP), amino resin particles of essentially uniformparticle size (narrow particle distribution) may not be obtained afterthe emulsion step and the curing step. That is, when the viscosity ofthe reaction solution is below 2×10⁻² Pa.s (20 cP), stability of theemulsion obtained in the emulsion step becomes poor. In this case,curing the amino resin precursor in the subsequent curing step oftenresults in enlarged amino resin crosslinked particles or aggregation ofparticles. That is, it becomes impossible to control particle size ofthe amino resin particles, with the result that only amino resincrosslinked particles of non-uniform particle size and wide particledistribution may be produced. Further, the poor stability of theemulsion may cause the particle size of the amino resin particles tovary from batch to batch during production. This variation often affectsin the product. On the other hand, a viscosity of the reaction solutionexceeding 5.5×10⁻² Pa.s (55 cP) in the temperature range of 95° C. to98° C. puts a load, for example, on a high-speed stirrer used in theemulsifying step and lowers the shearing force, with the result that thereaction solution cannot be emulsified sufficiently. In this case, itbecomes impossible to control particle size of the amino resinparticles, with the result that amino resin particles of non-uniformparticle size and wide particle distribution are usually obtained.

In the emulsifying step, the reaction solution containing the aminoresin particles is mixed with an aqueous solution of an emulsifier toemulsify, so that an emulsion of an amino resin precursor can beobtained. Emulsifiers that can be used in the aqueous solution are notparticularly limited as long as protective colloids are formed. Forexample, polyvinyl alcohol, carboxymethyl cellulose, sodium alginate,polyacrylic acid, water-soluble polyacrylate, and polyvinyl pyrrolidonecan be used. More specifically, the emulsifier is a water-solublepolymer emulsifier, or more preferably, a water-soluble polymer that canform protective colloids. The emulsifier is used in the form of anaqueous solution by being entirely dissolved in water, or by beingpartially dissolved in water and partially maintaining its original form(e.g., powder, granule, liquid, etc.). Among the foregoing emulsifiers,polyvinyl alcohol is preferable in terms of stability of the emulsionand interaction with the catalyst. The polyvinyl alcohol may besaponated either completely or partially. The degree of polymerizationis not particularly limited either. The larger the amount of emulsifierused with respect to the amino resin precursor, the smaller the particlesize of the product particles. It is preferable that the amount ofemulsifier used with respect to 100 percent by weight of the amino resinprecursor is in a range of 1 percent by weight to 30 percent by weight,or more preferably 1 percent by weight to 5 percent by weight. In theemulsion step, the reaction solution containing the amino resinprecursor may be added to the aqueous solution of an emulsifier and/oran aqueous solution of a surfactant, provided that it does not pose anyproblem. Further, an emulsifier and/or an aqueous solution of asurfactant may be added to the reaction solution containing the aminoresin precursor. In order to more efficiently carry out the emulsionstep, the reaction solution containing the amino resin precursor may beadded to an aqueous solution of an emulsifier and/or an aqueous solutionof a surfactant.

In a more preferred embodiment, in the emulsifying step, the reactionsolution of the amino resin precursor is added to an aqueous solution ofan emulsifier, so that the concentration of the amino resin precursor(i.e., concentration of the solid component) is in a range of 30 percentby weight to 60 percent by weight. Subsequently, the reaction solutionis emulsified in a temperature range of 70° C. to 100° C. Theconcentration of the aqueous solution of an emulsifier added is notparticularly limited and such a concentration is selected that theconcentration of the amino resin precursor can be adjusted within theforegoing range. The aqueous solution of the emulsifier maybe added tothe reaction solution of the amino resin precursor. A method of stirringin the emulsifying step should preferably use a device that is capableof powerful stirring, so that the emulsion of the amino resin precursorcan be obtained in the form of sufficiently small particles. Forexample, a method using a so-called high-speed stirrer or a homo mixeris preferable. When the concentration of the amino resin precursor fallsbelow 30 percent by weight, productivity of the amino resin crosslinkedparticles suffers. On the other hand, when the concentration of theamino resin precursor exceeds 60 percent by weight, the product aminoresin particles may enlarge or particles may aggregate. That is, itbecomes impossible to control particle size of the amino resinparticles, with the result that only amino resin particles ofnon-uniform size (wide particle distribution) are obtained. Specificexamples of the high-speed stirrer or homomixer include the homomixer orTK homomixer (the product of Tokushu Kika Kogyo Co., Ltd., Japan;provided with turbine-like shaped blades), the high-speed disper,homodisper, or TK lab disper (the product of Tokushu Kika Kogyo Co.,Ltd., Japan; provided with turbine-like shaped blades), the EBARA Milder(the product of Ebara manf. Co., Ltd.; provided with Sutto turbine-likeshaped blades), the high-pressure homogenizer (the product of Izumi FoodMachinery Co., Ltd.), and a static mixer.

In the present invention, in order to ensure that the amino resincrosslinked particles do not aggregate firmly, inorganic particles maybe optionally added to the emulsion. Specific examples of inorganicparticles include silica particulate, zirconia particulate, aluminumparticulate, alumina sol, and ceria sol. The inorganic particlespreferably has a specific surface area in a range of 5 m²/g to 400 m²/gand a particle size of no larger than 0.05 μm. These ranges of aspecific surface area or a particle size are even more effective inpreventing aggregation of amino resin crosslinked particles.

Specifically, the inorganic particles may be added to the emulsion, forexample, either directly in the form of particles, or in the form of adispersion in water. The amount of inorganic particles added to theemulsion is not particularly limited but a range of 1 percent by weightto 15 percent by weight with respect to 100 percent by weight of theamino resin precursor is preferable.

In the curing step, the amino resin particles (suspension containingamino resin particles) can be obtained by adding a catalyst to theemulsion of the amino resin precursor and curing the amino resinprecursor in an emulsion state. The catalyst (curing catalyst) ispreferably an acid. Examples of the acid include mineral acids such ashydrochloric acid, sulfuric acid (concentrated sulfuric acid), andphosphoric acid; ammonium salts of these mineral acids; sulfonic acidssuch as sulfamic acid, benzenesulfonic acid, paratoluene sulfonic acid,and dodecylbenzene sulfonic acid; and organic acids such as phthalicacid, benzoic acid, acetic acid, propionic acid, and salicylic acid. Ofthese catalysts (acids), mineral acids are preferable in terms of curingrate, and sulfuric acids are more preferable considering corrosion ofthe device and safety over the use of mineral acids. Using sulfuricacids rather than other acids such as dodecylbenzene sulfonic acid ispreferable because they impart no color on the amino resin crosslinkedparticles and they provides high solvent resistance to the amino resincrosslinked particles. These catalysts may be used either individuallyor a combination of two or more kinds. It is preferable that thecatalyst is added in an amount of 0.1 to 5 parts by weight with respectto 100 parts by weight of the amino resin precursor, or in an amount ofnot less than 0.002 mole, preferably not less than 0.005 mole, and morepreferably 0.01 to 0.1 mole, with respect to 1 mole of the aminocompound. An amount of catalyst exceeding 5 parts by weight destroys anemulsion state and causes the particles to aggregate. On the other hand,an amount of catalyst less than 0.1 part by weight extends the reactiontime or results in insufficient curing.

The reaction temperature for curing the emulsion is preferably in arange of 15° C. to 100° C. An end point of the reaction can be found bysampling or by visual inspection to see if amino resin particles haveformed. More preferably, the temperature of the emulsion is lowered toaround room temperature before gradually increasing it to cure theemulsion of the amino resin precursor and obtain the suspensioncontaining the amino resin particles. Specifically, a reaction time is 3to 15 hours.

The average particle size of the amino resin particles thus obtained ispreferably in a range of 0.05 μm to 30 μm, or more preferably 0.1 μm to15 μm. With the producing process according to the present invention,the standard deviation of average particle size can be controlled withina range of not more than 6 μm, preferably not more than 4 μm, or morepreferably not more than 2 μm.

In the producing process of amino resin crosslinked particles accordingto the present invention, after the condensation and curing of the aminoresin precursor to prepare amino resin particles, a suspension of theamino resin particles is neutralized (neutralizing step). Theneutralizing step removes the acid catalyst remaining in the suspension.More specifically, by neutralizing the acid catalyst, discoloring of theamino resin crosslinked particles after the heating step can besuppressed. As the term is used herein, “neutralize” in the presentinvention means adjusting the pH of the suspension, which is in a rangeof 1.5 to 3 after the curing catalyst is added and the amino resinprecursor is cured, to a pH of not less than 5, or more preferably 5 to9. The pH of the suspension is adjusted in this range by measuring itusing a pH meter or the like.

When the pH of the suspension is less than 5, the acid catalyst remains.This is not preferable because it imparts color to the amino resincrosslinked particles in a later heating step. That is, by adjusting thepH of the suspension to be not less than 5, or more preferably in arange of 5 to 9, it is possible to obtain amino resin crosslinkedparticles with high degree of hardness and superior solvent resistanceand superior heat resistance, without discoloring. As a neutralizer forneutralizing the suspension, alkaline compounds are preferable. Examplesof alkaline compounds include sodium carbonate, sodium hydroxide,potassium hydroxide, and ammonia. Among these alkaline compounds, sodiumhydroxide is preferable in terms of ease of handling, and an aqueoussolution of sodium hydroxide is suitably used.

The amino resin crosslinked particles can be extracted from thesuspension (reaction solution) by any separation method (separationstep). A filtration method or a method using a separator such ascentrifugal separator can be conveniently used. Note that, the aminoresin crosslinked particles extracted from the suspension may beoptionally washed by water or with other washing agents.

The amino resin crosslinked particles extracted in the separation stepis heated in a temperature range of 130° C. to 230° C. (heating step).The heating step removes moisture on the amino resin particles andremoves remaining unreacted formaldehyde, in addition to furtherpromoting condensation (crosslinking) of the amino resin particles. Aheating temperature below 130° C. in the heating step is not preferablebecause it is insufficient to properly condense (crosslink) the aminoresin particles and causes the degree of hardness, heat resistance, andsolvent resistance of the amino resin crosslinked particles to degrade.On the other hand, a heating temperature above 230° C. in the heatingstep is not preferable either because it may cause discoloring on theamino resin crosslinked particles. That is, by carrying out the heatingstep in the foregoing temperature range, it is possible to obtain aminoresin crosslinked particles with high degree of hardness and superiorsolvent resistance and superior heat resistance, without discoloring.

In order to obtain amino resin crosslinked particles with lessdiscoloring, it is preferable that the heating step be carried out underspecific conditions, in addition to the neutralizing step. Specifically,the heating step should preferably be carried out under an inert gasatmosphere with an oxygen concentration of not more than 10 percent byvolume. The oxygen concentration is preferably not more than 10 percentby volume, more preferably not more than 5 percent by volume, and evenmore preferably not more than 3 percent by volume. When the heating stepis carried out under an inert gas atmosphere with an oxygenconcentration of greater than 10 percent by volume, severe discoloringis caused on the amino resin crosslinked particles and intendedproperties of the amino resin crosslinked particles may not be obtained.It is therefore possible, by carrying out the heating step under aninert gas atmosphere with an oxygen concentration of not more than 10percent by volume, to further suppress discoloring of the amino resincrosslinked particles. Note that, an inert gas atmosphere with an oxygenconcentration of not more than 10 percent by volume is an atmospherewith an oxygen proportion of not more than 10 percent by volume and aninert gas proportion of not more than 90 percent by volume with respectto the entire atmosphere (gas). Examples of the inert gas includenitrogen gas, helium gas, and argon gas. Among these inert gases,nitrogen gas is preferable in terms of cost. Note that, in the followingexplanations, the term “heat treatment step” may be used to refer to aheating step in which heating is carried out under an inert gasatmosphere and in a temperature range of 130° C. 230° C.

In the case where the heating step is carried out for a number of times,for example, at different temperatures, at least one of these heatingsteps are preferably carried out under an oxygen concentration of notmore than 10 percent by volume. It is more preferable that all heatingsteps be carried out under an oxygen concentration of not more than 10percent by volume.

A method of heating is not particularly limited. For example, theheating step is finished when the moisture content of the amino resincrosslinked particles becomes not more than 3 percent by weight or less.

The average particle size of the amino resin crosslinked particles thusobtained is preferably in a range of 0.05 μm to 30 μm, or morepreferably 0.1 μm to 15 μm. With the producing process of amino resincrosslinked particles according to the present invention, the standarddeviation of the amino resin crosslinked particles can be controlled tobe not more than 6 μm, preferably not more than 4 μm, and morepreferably not more than 2 μm.

Further, by optionally carrying out steps of pulverizing, crushing,and/or classifying after the heating step, particles with an averageparticle diameter of not more than 10 μm, i.e., fine particles can beobtained. More specifically, particles with an average particle size of0.01 μm to 10 μm, or more preferably 0.1 μm to 10 μm can be obtained.The amino resin crosslinked particles obtained by the producing processaccording to the present invention hardly aggregate. Therefore, only asmall load is required to sufficiently pulverize the particles in thepulverizing step when it is carried out. Note that, the amino resincrosslinked particles will not be discolored in response to heat aseasily as amino resin crosslinked particles of a conventional producingprocess because the remaining acid catalyst is neutralized. It istherefore a preferred embodiment to provide amino resin crosslinkedparticles having desirable discoloring resistance and a small averageparticle size.

The producing process of amino resin crosslinked particles according tothe present invention, with the neutralizing step, can omit aconventional washing step for treating (e.g., removing) the acidcatalyst. Thus, the producing steps of the amino resin crosslinkedparticles are simpler, in addition to being more economical because lessdrainage is produced over conventional washing steps. Further, theproblem of discoloring on amino resin crosslinked particles in theheating step (heating treatment step), which was caused conventionallyby a failure to remove the acid catalyst sufficiently by washing, can besolved by the provision of the neutralizing step in the producingprocess of amino resin crosslinked particles according to the presentinvention, by which the acid catalyst, which is the cause ofdiscoloring, can be removed.

Color characteristics of the colorless amino resin crosslinked particlesof the present invention can be more preferably evaluated by measuring achange of Hunter whiteness in a heat discoloring test that is carriedout at 200° C. for 30 minutes, so as to assess whether the heatresistance of the colorless amino resin crosslinked particles isdesirable. Thus, it is preferable in the colorless amino resincrosslinked particles of the present embodiment that a change of Hunterwhiteness in a 200° C.×30 minutes heat discoloring test is within 15,more preferably within 10, and most preferably within 5. Amino resincrosslinked particles that are produced via a conventional step in whichthe acid catalyst is not removed (e.g., a step that does not neutralizethe acid catalyst) discolor from white to yellow in a heat discoloringtest, owning to the fact that the acid is remaining. That is, a Hunterwhiteness drops in a 200° C.×30 minutes pyrolysis test and a change ofHunter whiteness exceeds 15. In other words, resistance to heatdiscoloration becomes poor. The acid catalyst can be desirably removedby first neutralizing the acid catalyst used in the curing step of thepresent invention, followed by a heating step. Note that, in evaluatingcolor characteristics of the colorless amino resin crosslinked particlesof the present invention, a change of Hunter whiteness in a 200° C.×30minutes heat discoloring test can be preferably used for the evaluation,in addition to the foregoing evaluation of Hunter whiteness described inthis embodiment. Further, the embodiment described thus far isessentially colorless or white amino resin crosslinked particles,including those particles that contain a fluorescent brightener, ananti-oxidant, and the like.

The amino resin crosslinked particles produced by the producing processaccording to the present invention has superior solvent resistance andsuperior heat resistance and high degree of hardness, withoutdiscoloring. This makes the amino resin crosslinked particles suitablefor flatting agents; light diffusing agents; polishing agents; coatingagents for various films;

fillers such as polyolefin (e.g., polyethylene or polypropylene),polyvinyl chloride, various types of rubbers, paints, and toners;rheology control agents, and coloring agents, for example.

Second Embodiment

Another embodiment of the present invention is described below. Aminoresin crosslinked particles according to the present embodiment are theproduct of a condensation reaction between an amino compound andformaldehyde and are colored, wherein an area ratio in a solid-state¹³C-NMR analysis of a carbon atom signal that originates from an—NH—CH₂—NH— bond (C(II) bond) to a carbon atom signal that originatesfrom an —NH—CH₂O—CH₂—NH— bond (C(I) bond) is not less than 2, and acolor difference in a heat discoloring test is not more than 15.

Note that, for convenience of explanation, elements or configurationsalready discussed in the First Embodiment will not be described again.

The amino resin crosslinked particles according to the Second Embodimentare colored with a dye and/or a pigment. Amino resin has superioraffinity for a dye in particular. It is therefore preferable that theamino resin crosslinked particles are colored with a dye. Note that, inthe Second Embodiment, a color difference in a heat discoloring test,instead of the Hunter whiteness of the First Embodiment, is defined as aproperty of the amino resin crosslinked particles.

The dye may be water-soluble or oil-soluble, examples of which includewater-soluble monoazo dyes, water-soluble polyazo dyes, metal-containingazo dyes, dispersed azo dyes, anthraquinone acid dyes, anthraquinone vatdyes, indigo dyes, sulfide dyes, phthalocyanine dyes, diphenylmethanedyes, triphenylmethane dyes, nitro dyes, nitroso dyes, thiazol dyes,xanthene dyes, acridine dyes, azine dyes, oxazine dyes, thiazine dyes,benzoquinone dyes, naphthoquinone dyes, and cyanine dyes.

Examples of pigments include azo dyes such as fast yellow, disazoyellow, disazo orange, naphthol red, and pigment orange; organicpigments such as phthalocyanine blue, as phthalocyanine green,indanthrene blue, flavanthrone, dibromoanzanethrone, anthrapyrimidine,quinacridone, isoindolynone, thioindigo, perylene, and dioxadine;titanium oxide, iron oxide, zinc oxide, barium sulfate, calcium sulfate,barium carbonate, calcium carbonate, magnesium carbonate, talc, clay,and carbon black. These dyes and/or pigments may be used eitherindividually or in a combination of two or more kinds, depending on theintended color of the amino resin crosslinked particles. Further, notlimiting to dyes or pigments, any additive for the purpose of impartingcolor can be added to the colored amino resin crosslinked particles ofthis embodiment.

It is preferable that the colored amino resin crosslinked particlesaccording to the present embodiment is colored with a fluorescent dye.The fluorescent dye re-radiates part of absorbed light energy in theform of fluorescent light having a longer wavelength than the incidentwave. Therefore, the fluorescent dye has higher reflectance thanordinary dyes and imparts a highly photoluminescent color.

Examples of the fluorescent dye include Fluorescent Red 632 andFluorescent Yellow 600 (products of Arimoto Chemical Co., Ltd.),Rhodamine B and Rhodamine 6GCP (the products of Sumitomo Chemical Co.,Ltd.), Quinoline Yellow SS-5G and Quinoline Yellow GC (the products ofChuo Gousei Kagaku Co., Ltd.), Azosol Brilliant Yellow 4GF, Azosol FastBlue GLA, Cellitone Pink 3B, Fast Yellow YL, Victoria Blue FN, BrilliantSulfoflavin FF, thioflavin, Basic Yellow HG, fluorescein, and eosin.

These fluorescent dyes may be used individually or in a mixture of twoor more kinds, depending on the intended color of the amino resincrosslinked particles.

Further, the fluorescent dye may be used in mixture with the dye and/orpigment.

In the Second Embodiment, the amino resin crosslinked particles of thepresent embodiment are rendered fluorescent by being colored with afluorescent dye, and has a heat discoloring resisting property. Notethat, in the following explanations, the “dye” includes a fluorescentdye.

It is preferable that the colored amino resin crosslinked particlesaccording to the Second Embodiment have a color difference of not morethan 15, or more preferably not more than 10, in a heat discoloringtest. It is even more preferable, in addition to these preferable rangesof color difference, that a change in b value (Δb*) before and after theheat discoloring test is not more than 10. It is most preferable that acolor difference and a change in b value (Δb*) before and after the heatdiscoloring test are both not more than 10.

In the heat discoloring test, the product colored amino resincrosslinked particles are allowed to stand for 30 minutes in a constanttemperature device maintained at 200° C. A color difference iscalculated from L value (index of brightness) and a value and b value(indices of chromaticness) before and after the heat discoloring test,which are measured using a spectrophotometer. More specifically, thecolored amino resin crosslinked particles measured here are the productof the foregoing producing process with an additional ordinarypurification step. In order to have a uniform temperature in theconstant temperature device, a heat is applied to a predetermined amountof the colored amino resin crosslinked particles that are spread over astainless steel bat.

A color difference is a ΔE*_(ab) in the color system L*, a*, b*, whichis calculated as follows.ΔE* _(ab)=[(ΔL*)²+(Δa)²+(Δb*)²]^(1/2)where ΔL*, Δa*, and Δb* are changes in their values before and after theheat discoloring test.

A color difference above 15 is not preferable because the colored aminoresin crosslinked particles discolor. Note that, as the term is used inthe present embodiment, a color difference that exceeds 15 in the heatdiscoloring test of the product amino resin crosslinked particles isregarded as “discoloring”.

As described, it is preferable in the amino resin crosslinked particlesaccording to the Second Embodiment, which are colored particles, that(a) a color difference of a heat discoloring test is not more than 15,and an area ratio in a solid-state ¹³C-NMR analysis of a carbon atomsignal that originates from the C(II) bond to a carbon atom signal thatoriginates from the C(I) bond is not less than 2, (b) a color differenceof a heat discoloring test is not more than 15, and an amount offormaldehyde generated is not more than 1000 ppm in the pyrolysis test,and (c) a color difference of a heat discoloring test is not more than15, and an area ratio in a solid-state ¹³C-NMR analysis of a carbon atomsignal that originates from the C(II) bond to a carbon atom signal thatoriginates from the C(IV) bond is not less than 0.20.

The following describes a producing process of colored amino resincrosslinked particles according to the Second Embodiment. The producingprocess of colored amino resin crosslinked particles according to theSecond Embodiment includes the steps of: (1) adding a catalyst to anemulsion that was obtained by mixing an amino resin precursor, which isthe product of a reaction between an amino compound and formaldehyde,with an aqueous solution of an emulsifier, so as to cure the amino resinprecursor to prepare amino resin particles; (2) neutralizing asuspension that contains the amino resin particles obtained in step (1);and (3) heating the amino resin particles after step (2) in atemperature range of 130° C. to 230° C., wherein the step (1) ofpreparing the amino resin particles includes the step of coloring theamino resin precursor with a dye and/or a pigment.

Specifically, for example, a catalyst is added to an emulsion of anamino resin precursor, a dye, and an emulsifier, which is obtained by areaction of the amino compound and formaldehyde, so as to cure the aminoresin precursor and obtain amino resin particles (suspension containingamino resin particles). Thereafter, the remaining acid catalyst isneutralized by adding an alkali to adjust the pH of the suspensioncontaining the amino resin particles, which is in a range of 1.5 to 3,within a range of not less than 5, or more preferably 5 to 9. Afterfiltration, the amino resin particles are heated in a temperature rangeof 130° C. to 230° C. The heating temperature is preferably in a rangeof 130° C. to 210° C., or more preferably 130° C. to 190° C. The pH ofthe suspension is determined using a pH meter or the like.

A timing of adding a dye is described below. A timing of adding a dye isnot particularly limited and a dye can be added at any stage in the stepof preparing the amino resin particles, i.e., in the reaction step,emulsifying step, or curing step. In order to attain uniform color, adye should be added in the reaction step. It is preferable that a dye beadded, for example, in the form of a dispersion in water or an aqueoussolution.

A dye added in the step of coloring the reaction solution (solution)according to the foregoing method of addition is an oil-soluble dye. Thetype of oil-soluble dye is not particularly limited. Specific examplesinclude solvent-soluble dyes such as Oil Orange B and Oil Blue BA (theproducts of Chuo Gousei Kagaku Co., Ltd.), Azosol Brilliant Yellow 4GF,Azosol Fast Blue GLA, and Oil Red TR-71; and dispersive dyes such asFast Yellow YL, Fast Blue FG, Cellitone Pink FF3B, and Cellitone Pink3B. These dyes may be used individually or in a mixture of two or morekinds.

The amount of a dye contained in the dispersion is not particularlylimited, and a range of 1 percent by weight to 50 percent by weight, ormore preferably 20 percent by weight to 40 percent by weight ispreferable. When the content of a dye is less than 1 percent by weight,a large volume of dispersion needs to be added, which might lowerproductivity of the amino resin crosslinked particles. On the otherhand, when the content of a dye exceeds 50 percent by weight, thedispersion becomes less fluidic. In this case, handling of the dye maybecome difficult and it might take more effort to add the dye. Whenusing an oil-soluble dye, a dispersion auxiliary agent may be optionallyused when the dye is dispersed in water, because wettability ofoil-soluble dyes in water is poor. Note that, a method of preparing adispersion of a dye in water and a method of adding and mixing thedispersion with the reaction solution are not particularly limited.

The reaction solution (solution) with the dye dispersion is adjusted tohave a pH of 6 to 12, or more preferably 7 to 9, using an alkalineagent, for example, such as sodium carbonate, sodium hydroxide,potassium hydroxide, or ammonium water. This enables the condensationand curing of the amino resin precursor in the curing step to besufficiently controlled. The amount of alkaline agent added is notparticularly limited. Further, the alkaline agent can be added and mixedin any form but the form of an aqueous solution is preferable.

The emulsion of the amino resin precursor can be obtained by adding andmixing the dye dispersion in the reaction solution containing the aminoresin precursor, and then by emulsifying the reaction solution, afteradjusting pH, in the presence of an emulsifier. The type of emulsifieris not particularly limited, and the emulsifier as described in theFirst Embodiment is used.

An additional step (second coloring step) may be provided to add a dyeto the emulsion produced in the emulsifying step. The dye shoulddissolve in water, i.e., water-soluble dyes are used. The type ofwater-soluble dye is not particularly limited. Examples of water-solubledyes include basic dyes such as Rhodamine B and Rhodamine 6GCP (theproducts of Sumitomo Chemical Co., Ltd.), Methyl Violet FN, and VictoriaBlue FN; and acidic dyes such as Quinoline Yellow SS-5G and QuinolineYellow GC (the products of Chuo Gousei Kagaku Co., Ltd.), Acid MagentaO, Methyl Violet FB, and Victoria Blue FB. These dyes may be usedindividually or in a mixture of two or more kinds. By the coloring step(first coloring step) of adding a dispersion of an oil-soluble solventin water and by the second coloring step, it is possible to obtain aminoresin crosslinked particles with more uniform color, i.e., particleswith improved color uniformity.

The concentration of the dye in an aqueous solution, when the dye isadded in the form of an aqueous solution in the second coloring step, isnot particularly limited, but a range of 0.1 percent by weight to 5percent by weight is preferable, and a range of 1 percent by weight to 3percent by weight is more preferable. When the concentration of the dyeis less than 0.1 percent by weight, a large volume of aqueous solutionneeds to be added, which might lower productivity of the amino resincrosslinked particles. On the other hand, when the concentration of thedye exceeds 5 percent by weight, the emulsion containing the dye becomesinstable, which might cause the amino resin crosslinked particles toenlarge or particles to aggregate. Note that, a method of preparing anaqueous solution of a dye in water and a method of adding and mixing theaqueous solution with the emulsion are not particularly limited.

Note that, the foregoing explanations, which were based on a dye, arealso applicable to a pigment and a fluorescent pigment. Accordingly, nofurther explanations will be given thereto.

The amino resin crosslinked particles according to the Second Embodimentmay also be colored with a white dye and/or a white pigment. In thiscase, the amino resin crosslinked particles satisfy at least one of (a)Hunter whiteness of not less than 85 percent and (b) color difference ofnot more than 15 in a heat discoloring test, in addition to theforegoing heat discoloring resisting property (area ratio of signals ina solid-state ¹³C-NMR analysis in the foregoing ranges).

The colored amino resin crosslinked particles thus obtained has anaverage particle size preferably in a range of 0.05 μm to 30 μm, or morepreferably 0.1 μm to 15 μm. With the producing process of amino resincrosslinked particles according to the present invention, the standarddeviation of the amino resin crosslinked particles can be controlled tobe not more than 6 μm, more preferably not more than 4 μm, and mostpreferably not more than 2 μm.

As described, the amino resin crosslinked particles of the presentinvention, which is produced by condensation of an amino compound andformaldehyde, are adapted so that an area ratio in a solid-state ¹³C-NMRanalysis of a carbon atom signal that originates from the —NH—CH₂—NH—bond to a carbon atom signal that originates from the —NH—CH₂O—CH₂—NH—bond is not less than 2, and a color difference of a heat discoloringtest is not more than 15.

As described, the colored amino resin crosslinked particles of thepresent invention, which is produced by condensation of an aminocompound and formaldehyde, are adapted so that the amount offormaldehyde generated is 1000 ppm in a pyrolysis test, and a colordifference of a heat discoloring test is not more than 15.

As described, the colored amino resin crosslinked particles of thepresent invention, which is produced by condensation of an aminocompound having a triazine ring and formaldehyde, are adopted so that anarea ratio in a solid-state ¹³C-NMR analysis of a carbon atom signalthat originates from the —NH—CH₂—NH— bond to a carbon atom signal thatoriginates from a triazine ring of the amino compound is not less than0.20, and a color difference of a heat discoloring test is not more than15.

The colored amino resin crosslinked particles of the present invention,with a color difference of not more than 15 in a heat discoloring testand with a 2.0 or greater area ratio in a solid-state ¹³C-NMR analysisof a carbon atom signal that originates from the —NH—CH₂—NH— bond to acarbon atom signal that originates from the —NH—CH₂O—CH₂—NH— bond,contain a small proportion of the —NH—CH₂O—CH₂—NH— bond. Thus, theproduct colored amino resin crosslinked particles, in addition to thedesirable heat resistance, generate less formaldehyde when heated.Further, because the colored amino resin crosslinked particles containmore —NH—CH₂—NH— bonds, the product amino resin crosslinked particleshave high degree of hardness and superior heat resistance and superiorsolvent resistance.

It is preferable in the colored amino resin crosslinked particles of theSecond Embodiment that the amino compound contains at least one kind ofcompound selected from the group consisting of benzoguanamine,cyclohexane carboguanamine, cyclohexene carboguanamine, and melamine, inan amount of 40 percent by weight to 100 percent by weight, and that themole ratio of the amino compound to the formaldehyde is in a range of1:1.5 to 1:3.5.

As described, the producing process of colored amino resin crosslinkedparticles of the Second Embodiment includes the steps of: (1) adding acatalyst to an emulsion that was obtained by mixing an amino resinprecursor, which is the product of a reaction between an amino compoundand formaldehyde, with an aqueous solution of an emulsifier, so as tocure the amino resin precursor to prepare amino resin particles; (2)neutralizing a suspension that contains the amino resin particlesobtained in step (1); and (3) heating the amino resin particles afterstep (2) in a temperature range of 130° C. to 230° C., wherein the step(1) of preparing the amino resin particles includes the step of coloringthe amino resin precursor with a dye and/or a pigment.

By thus neutralizing the suspension after the curing step, discoloringof the colored amino resin crosslinked particles, which might occur inthe heating step after the neutralizing step, can be suppressed. Inaddition, by heating the amino resin crosslinked particles at 130° C. ora greater temperature, water and remaining formaldehyde can be removed,in addition to promoting condensation. As a result, it is possible toprovide colored amino resin crosslinked particles having high degree ofhardness and superior solvent resistance and superior heat resistance,which generate almost no odor of formaldehyde when heated.

In the producing process of the present invention, the heating step thatis carried out in an atmosphere of inert gas can be suitably adaptedwhen the amino resin crosslinked particles, after separated from thereaction solution, are heated at a relatively high temperature (at least130° C.). Specifically, the producing process of the present invention,in the producing process of amino resin crosslinked particles with thecuring step using an acid catalyst and a heating step for heating aminoresin particles at a relatively high temperature after they wereseparated from the cured suspension, further includes a step ofneutralizing the acid catalyst; and/or a heating step that is carriedout in an atmosphere of inert gas.

Another embodiment of such an invention, with a slight modification tothe First and Second Embodiments, is described below.

Third Embodiment

In a producing process of amino resin crosslinked particles according tothe present embodiment, an initial condensate (amino resin precursor),having affinity to water, composed of melamine, benzoguanamine, andformaldehyde, or composed of melamine or benzoguanamine andformaldehyde, is condensed and cured in an aqueous solution containing asurfactant, in the presence of alkylbenzene sulfonic acid with an alkylgroup of 10 to 18 carbon atoms, so as to prepare a suspension of curedresin. The cured resin is then separated from the suspension and driedin an atmosphere of inert gas to obtain amino resin crosslinkedparticles. The amino resin crosslinked particles obtained according tothis process have a uniform particle size.

Note that, for convenience of explanation, elements or configurationsalready discussed in the First and Second Embodiments will not bedescribed again.

Specifically, the producing process of amino resin crosslinked particlesaccording to the present embodiment includes the steps of adding acatalyst to a mixture of an amino resin precursor, which is the productof a reaction of an amino compound and formaldehyde with an aqueoussolution of a surfactant, so as to cure the amino resin precursor andprepare amino resin particles; and heating the amino resin particles inan atmosphere of inert gas whose oxygen concentration is not more than10 percent by volume, and in a temperature range of 130° C. to 230° C.

The initial condensate, having affinity to water, composed of melamine,benzoguanamine, and formaldehyde, or composed of melamine orbenzoguanamine, and formaldehyde, used in the present embodiment is awater-soluble resin or a water-dispersive resin, which is obtained by areaction of melamine and/or benzoguanamine with formaldehyde, or areaction of a mixture of the two with formaldehyde, according to anordinary method. The degree of water affinity is generally decided bythe amount of water dropped, in percent by weight with respect to theinitial condensate, on the initial condensate maintained at 15° C. toturn the initial condensate turbid. (This measure of water affinity willbe referred to as miscibility with water.) A miscibility with watersuitable in the present embodiment is 100 percent or greater. An initialcondensate with a miscibility with water less than 100 percent producesa relatively non-uniform emulsion having large particle sizes, even whendispersed in an aqueous solution containing a surfactant, which makes itdifficult to obtain amino resin crosslinked particles with uniformparticle size.

The formaldehyde may be, for example, in the form of formalin, trioxane,or paraformaldehyde, having a capability to generate formaldehyde.

Specific examples of surfactant include anionic surfactants, cationicsurfactants, non-ionic surfactants, and amphoteric surfactants. Of thesesurfactants, anionic surfactants, non-ionic surfactants, or a mixture ofthese two surfactants are preferable.

Examples of anionic surfactants include alkali metal alkylsulfates suchas sodium dodecyl sulfate and potassium dodecyl sulfate; ammoniumalkylsulfates such as ammonium alkyl sulfate; sodium dodecyl polyglycolether sulfate; sodium sulforicinoate; alkali metal salts such assulfonated paraffin; alkylsulfonates, such as ammonium salts ofsulfonated paraffin; sodium laurate; triethanol amine oleate; fatty acidsalts such as triethanolamine abietate; alkylarylsulfonates, such asalkali metal sulfonates of sodium dodecylbenzene sulfonate or alkaliphenol hydroxyethylene; high alklylnaphthalenesulfonate; a condensate ofnaphthalene sulfonic acid and formalin; dialkylsulfosuccinate;polyoxyethylene alkylsulfate salt.

Examples of non-ionic surfactants include polyoxyethylene alkylether;polyoxyethylene alkylarylether; sorbitan fatty acid ester;polyoxyethylene sorbitan fatty acid ester; fatty acid monoglyceride suchas monolaurate of glycerol; co-polymer of polyoxyethylene andoxypropylene; and a condensate of ethyleneoxide and fatty amine, anamide, or an acid.

The amount of surfactant added is preferably in a range of 0.01 percentby weight to 10 percent by weight with respect to 100 percent by weightof the initial condensate. With an amount of surfactant less than 0.01percent by weight, a stable suspension of cured resin may not beobtained. On the other hand, with an amount of surfactant more than 10percent by weight, undesirable bubbles may form in the emulsion orsuspension. In other cases, the properties of the product amino resincrosslinked particles may be adversely affected.

In the present embodiment, the alkylbenzenesulfonic acid having an alkylgroup with 10 to 18 carbon atoms shows an unique surfactant action inthe aqueous solution of the initial condensate, and therefore is anindispensable component to produce a stable suspension of the curedresin. Examples of the alkylbenenesulfonic acid includedecylbenzenesulfonic acid, dodecylbenzenesulfonic acid,tetradecylbenzenesulforiic acid, hexadecylbenzenesulfonic acid, andoctadecylbenzenesulfonic acid. These alkylbenzenesulfonic acids may beused individually or in combination of two or more kinds.

The amount of alkylbenzenesulfonic acid used is preferably 0.1 percentby weight to 20 percent by weight, or more preferably 0.5 percent byweight to 10 percent by weight, with respect to 100 percent by weight ofthe initial condensate. When the amount of alkylbenzenesulfonic acid isless than 0.1 percent by weight, the reaction time of condensation andcuring is prolonged and a stable suspension of the cured resin may notbe obtained. As a result, the amino resin crosslinked particles mayaggregate to produce coarse particles. On the other hand, when theamount of alkylbenzenesulfonic acid is more than 20 percent by weight,the alkylbenzene sulfonic acid will be distributed in excess in thecured resin in the suspension, with the result that the cured resin isplasticized. This encourages the particles to aggregate or fuse duringcondensation and curing. As a result, the amino resin crosslinkedparticles may not be obtained in uniform particle size.

In the present embodiment, the initial condensate having affinity towater can be condensed and cured in an aqueous solution by adding thesurfactant and alkylbenzenesulfonic acid to the aqueous solution of theinitial condensate or to a white emulsion, which is a dispersion of theinitial condensate in water, and by maintaining the mixture withstirring at a temperature of 0° C. to a temperature greater than 100° C.under applied pressure.

The surfactant and alkylbenzenesulfonic acid can be added, for example,(1) by adding the initial condensate to an aqueous solution containingthe surfactant and alkylbenzenesulfonic acid, or (2) by adding thesurfactant and alkylbenzenesulfonic acid to an aqueous solutioncontaining the initial condensate.

The concentration of the initial condensate in the aqueous solution ispreferably in a range of 5 percent by weight to 20 percent by weight,taking into account ease of handling of the emulsion and cost ofoperation.

The reaction of condensation and curing is generally finished uponraising the temperature to a 90° C. or greater temperature andmaintaining this temperature for a certain period of time. However,high-temperature curing is not necessarily required and condensation mayonly be carried out at a low temperature and for a short period of time,provided that the cured resin in the suspension is cured to the degreewhere it does not swell by methanol or acetone.

The suspension of the cured resin so obtained contains a uniformparticle size and is highly stable without aggregation of particles. Thecured resin can be separated from the suspension of cured resin by aconventional method. Specifically, the cured resin can be separated fromthe suspension by various separation methods, such as separation bynatural precipitation, centrifugation, or decantation, or alternativelyseparation by filtration.

In the producing process of amino resin crosslinked particles accordingto the present embodiment, the cured resin separated from the suspensionis heated in an atmosphere of inert gas. That is, there is provided aheating step in an atmosphere of inert gas, i.e., a heat treatment stepis provided.

In the present embodiment, the heating step (heat treatment step) in anatmosphere of inert gas is a step in which the particles (cured resin)separated from the reaction solution (suspension) are heated in anatmosphere of inert gas when they are dried. By the heat treatment step,i.e., by heating the cured resin in an atmosphere of inert gas, thecured resin, in addition to being dried, can be further crosslinked bythe remaining curing catalyst. A detailed explanation in regard toheating in an atmosphere of inert gas will not be given here because itwas explained in the Second Embodiment. Specifically, the cured resinmay be dried in an atmosphere of inert gas by drying under reducedpressure or hot-air drying.

By drying the cured resin in an atmosphere of inert gas, discoloring ofthe amino resin crosslinked particles can be suppressed. Morespecifically, in the case of colorless amino resin crosslinkedparticles, the Hunter whiteness of the product amino resin crosslinkedparticles is 85 percent or greater. In the case of colored amino resincrosslinked particles, the color difference of the amino resincrosslinked particles before and after the heat discoloring test is notmore than 15.

Further, the separation can be facilitated by adding a coagulant, suchas aluminum sulfate, prior to the separation.

The dried cured resin can be crushed using, for example, a ball mill toobtain spherical and uniform amino resin crosslinked particles (narrowparticle distribution).

Note that, in the foregoing description, the “emulsion” refers to anemulsion and/or a suspension.

The cured resin (amino resin particles) can be obtained in the describedmanner, for example, (X) by mixing, when using a water-soluble rawmaterial such as melamine, dodecylbenzenesulfonic acid with theresulting amino resin precursor (initial condensate), so as to condense,cure, and thereby deposit amino resin particles, or (Y) by condensingand curing, when using a water-insoluble raw material such asbenzoguanamine, the resulting amino resin precursor (initial condensate)in a water-dispersed state and in the presence of a catalyst, so as toobtain amino resin particles.

In summary, the producing process with the step of neutralizing the acidcatalyst is applicable to producing processes of amino resin crosslinkedparticles when the processes use at least the acid catalyst and obtain asuspension of amino resin particles.

Further, the producing process with the step of further heating theamino resin particles in an atmosphere of inert gas after separating theamino resin particles from the suspension and heating them at arelatively high temperature (130° C. or greater) is applicable to anyproducing process if the process includes at least the step ofseparating the amino resin particles from the suspension after curingand subsequently heating the amino resin particles. That is, theproducing process of the present invention includes the step ofneutralizing the acid catalyst, and/or the step of heating the aminoresin particles in an atmosphere of inert gas, in addition to the curingstep using an acid catalyst, and the heating step, in which the aminoresin particles separated from the suspension after curing is heated ata relatively high temperature (130° C. or greater). The substance of theforegoing process is therefore applicable to all of the foregoing First,Second, and Third Embodiments. The producing process of amino resincrosslinked particles is also applicable to other embodiments within themodification range of the present invention. Such embodiments,obviously, are also within the scope of the present invention.

EXAMPLES

The present invention will be described in more detail by way ofExamples and Comparative Examples. The product amino resin crosslinkedparticles were analyzed under the following measurement conditions,using a solid-state ¹³C-NMR device (BRUKER AVANCE 400, the product ofBruker).

The measurement was carried out under the following conditions:

Probe used: 4 mm MAS400;

Target nucleus: ¹³C;

Resonance frequency of target nucleus: 100.63 MHz;

90° pulse of target nucleus: 4.0 μs;

Pulse Program: DD/MAS (dipole decoupling method);

Pulse width of target nucleus: 1.5 μs;

Recurrent time: 40 seconds;

Number of integration: 4096;

Temperature of observation: 300 K;

Chemical shift value of reference material: Glycine (176.03 ppm, 44.02ppm)

Various properties of the product amino resin crosslinked particles weremeasured in the manner described below.

A Hunter whiteness was measured in accordance with the JIS standardP8123, using a spectrocalorimeter (SZ-Σ80, Nippon Denshoku Kogyo Co.,Ltd.).

A color difference of colored amino resin crosslinked particles wasmeasured as follows.

3 ml of ethylene glycol was placed in a plastic bag containing 2.00 g ofa sample. After uniformly dispersing the sample therein, the sample wastransferred onto glass cells (calorimeter Σ80 cell; 30 mm diameter). Theglass cells with the evenly dispersed sample was then placed in acalorimeter (Macbeth color eye 3000, the product of SAKATA INX), so asto measure color difference. As a standard white board, a ceramiccalibration tile for Macbeth color eye 3000 was used. Note that, afirst-rate reagent was used for the ethylene glycol.

Solvent resistance was measured as follows. 10 ml of acetone and 0.5 gof amino resin crosslinked particles of an Example or a ComparativeExample were added into a glass container to prepare a dispersion. Afterstirring the dispersion for 1 minute at 300 rpm and 25° C., thedispersion was filtered out through a filter paper (Toyo Filter Paper,Co., Ltd., No. 2). The surface morphology of the amino resin crosslinkedparticles remaining on the filter paper was observed under a microscope(at 400 times magnification). Evaluations were made such that aggregatedcoarse amino resin crosslinked particles were denoted as “x”, andnon-aggregated and non-coarse amino resin crosslinked particles weredenoted as “∘”. The surface of amino resin crosslinked particles withlow (poor) solvent resistance is eroded by the solvent and becomessticky, with the result that the amino resin crosslinked particlesaggregate to produce coarse particles.

Light resistance was measured as follows. A 150 ml glass container wascharged with amino resin crosslinked particles of an Example or aComparative Example, 60 g of 3 φ glass beads, 8 g of vinyl chloride sol,and 30 g of toluene, so as to prepare a reagent. The mixture was thenshaken for 30 minutes using a paint shaker (PAINT SHAKER, the TOYOSEIKIproduct). The vinyl chloride sol is a mixture of 49.5 percent by weightof vinyl chloride paste resin (provided by (Kaneka Corporation), 49.5percent by weight of dioctylphthalate, and 1 percent by weight of avinyl chloride stabilizer (TMF-380G, the product of Tokyo Fine ChemicalCo., Ltd.). After shaking, an suitable amount of sample was placed on aBoron-Kent paper and evenly spread over the Boron-Kent paper with a barcoater No. 12. Then, a portion of the Boron-Kent paper coated with thesample was covered with an aluminum foil and UV light was projectedthereon for 5 hours at an UV intensity of 100 mW/cm², using a UV lightirradiating device (EYE SUPER UV TESTER, model SUV-F1, provided byIwasaki Electric Co., Ltd.). Then, changes in color of the Boron-Kentpaper irradiated with the UV light and not irradiated with the UV light(covered with the aluminum foil) were observed by visual inspection.Evaluations were made such that discoloring was denoted as “x” and nodiscoloring was denoted as “∘”.

The amount of formaldehyde (“HCHO” hereinafter) generated was measuredby gas chromatography by measuring the amount of formaldehyde generatedwhen 1 mg of the product amino resin crosslinked particles were heatedat 160° C. The odor given off by the amino resin crosslinked particlesheated at 160° C. was also confirmed. The measurement conditions of gaschromatography (pyrolytic device JHP-2, the product of Japan AnalyticalIndustry Co., Ltd.; gas analyzer GC-14A (detector: TCD), the product ofShimadzu Corporation) were such that the initial temperature ofpyrolysis was 160° C. and the duration of pyrolysis was 5 seconds, usinga measurement column (APS-201 Flusin T, 20%, 60/80 mesh, 3.2 φ×3.1 m).

The heat discoloring test was carried out in the following manner. 100 gof dried amino resin crosslinked particles (colorless or colored), whichwere spread in thin layer over a 50 cm×50 cm stainless bat, were placedin a constant temperature device of 200° C. for 30 minutes. Air wascirculated in the constant temperature device. A change of Hunterwhiteness and a color difference were measured in the heat discoloringtest to evaluate resistance to heat discoloration.

An average particle size was measured using the Coulter Multisizer TypeII, the product of Coulter Inc. Note that, any particle size measuringdevice of equivalent performance and equivalent standard may be used tomeasure an average particle size.

The following Examples 1 through 6 are based on a process that comprisesthe steps of (1) adding a catalyst to an emulsion of an amino resinprecursor, the emulsion being a mixture of an amino resin precursor,which is the product of a reaction of an amino compound withformaldehyde, and an aqueous solution of the emulsion, so as to cure theamino resin precursor in an emulsion state and obtain a suspension ofthe amino resin precursor; (2) neutralizing the suspension of step (1)containing amino resin particles; and (3) heating the amino resinparticles in a temperature range of 130° C. to 230° C. That is, thefollowing Examples 1 through 6 do not limit the present invention in anyways.

Further, the Examples 1 through 3 and Comparative Examples 1 and 2correspond to the First Embodiment.

The Examples 4 through 6 and Comparative Examples 3 and 4 correspond tothe Second Embodiment.

The Examples 7 through 9 and Comparative Example 5 correspond to theThird Embodiment.

Note that, it is assumed in the following Examples and ComparativeExamples that the heating step (heat treatment step) is carried out inair unless otherwise noted.

Example 1

A 10 L reactor equipped with a stirrer, a reflux condenser, athermometer, and a vibration viscometer (product of MIVI ITS JAPAN;Model No. MIVI 6001) was charged with 3000 g (16 moles) ofbenzoguanamine as an amino compound, 2600 g of 37 percent by weightformalin (32 moles of formaldehyde), and 10 g of an aqueous solution of10 percent by weight sodium carbonate (0.01 mole of sodium carbonate).The temperature of the reaction mixture was raised with stirring to 95°C. to start a reaction.

The reaction was ended by cooling the reaction solution when theviscosity of the reaction mixture became 4.5×10⁻² Pa.s (45 cP) (5 hoursinto the reaction). The result was a reaction solution containing anamino resin precursor, which is the initial condensate of benzoguanamineand formaldehyde (reaction step).

Then, an aqueous solution dissolving 120 g of polyvinyl alcohol (productname PVA 205, provided by Kuraray Co., Ltd.) as an emulsion in 2050 g ofwater was charged in a 15 L reactor equipped with a reflux condenser, ahomomixer (stirrer, provided by Tokushu Kika Kogyo Co., Ltd.), and athermometer, etc. The temperature of the mixture was raised to 75° C.with stirring. After adding the reaction solution to the reactor, thetemperature of the reaction mixture was raised to 77° C. and the contentwas vigorously stirred at 7000 rpm at a maintained temperature of 77°C., so as to emulsify the amino resin precursor and obtain an emulsionthat contained the amino resin precursor in a concentration of 52.5percent by weight (emulsifying step). A measurement of the emulsion withthe multisizer showed that the amino resin precursor in the emulsion hadan average particle size (d50) of 2.4 μm and a standard deviation of 0.7μm. The emulsion so obtained was cooled to 30° C. To the reactor wasthen added, as a water dispersion of silica provided as an inorganiccompound, 3000 g of AEROSIL 200 (provided by Japan Aerosil) with a solidconcentration of 10 percent by weight. Thereafter, the content wasstirred with the homomixer for 5 minutes at 4000 rpm.

Then, an aqueous solution dissolving 40 g (0.4 mole) of concentratedsulfuric acid as a catalyst in 1200 g of pure water was added to theemulsion (at a content temperature 30° C.), and the temperature of thereaction mixture was raised with stirring to 90° C. at 10° C./hr. Thereaction mixture was maintained at 90° C. for an hour to condense andcure the amino resin precursor (curing step). Thus, the total reactiontime was 7 hours.

After the curing step, the suspension containing amino resin particleswas cooled to 30° C., and the pH of the suspension was adjusted to 7.5,using 5 percent by weight sodium hydroxide aqueous solution(neutralizing step). After the neutralizing step, the amino resinparticles were separated from the suspension by filtration. The aminoresin particles thus separated was heated for 3 hours at 150° C.(heating step) and gently crushed with a pestle in a mortar. The resultwas a white powder of amino resin crosslinked particles. Measurements ofthe amino resin crosslinked particles with the multisizer showed thatthe amino resin crosslinked particles had an average particle size (d50)of 2.7 μm and a standard deviation of 0.8 μm. Then, a solid-state¹³C-NMR analysis of the amino resin crosslinked particles was carriedout, and Hunter whiteness, solvent resistance, light resistance, and anamount of HCHO generated in a pyrolysis test were measured. In addition,odor was confirmed. The main reaction conditions and results aresummarized in Table 1. The results of solid-state ¹³C-NMR analysis onthe amino resin crosslinked particles are shown in FIG. 1. In thesolid-state ¹³C-NMR analysis, the resulting spectrum had a C(I) derivedcarbon atom signal in a 60 ppm to 90 ppm range, a C(II) derived carbonatom signal in a 30 ppm to 70 ppm range, a C(III) derived carbon atomsignal in a 110 ppm to 150 ppm range, and a C(IV) derived carbon atomsignal in a 155 ppm to 190 ppm range.

Example 2

A reaction step was carried out in the same manner as in Example 1. Thereaction was ended when the viscosity became 5.5×10⁻² Pa.s (55 cP) (5.5hours into the reaction), so as to obtain an amino resin precursor.Then, an emulsifying step was carried out as in Example 1 to obtain anemulsion that contained the amino resin precursor in a concentration of52.5 percent by weight. The amino resin precursor in the emulsion had anaverage particle size 2.6 μm and a standard deviation 1.01 μm. Theemulsion so obtained was cooled to 30° C. To the reactor was then added,as a water dispersion of alumina provided as an inorganic compound, 3000g of Aluminum Oxide C (product of Japan Aerosil) with a solidconcentration of 10 percent by weight. The content of the reactor wasstirred for 5 minutes at 4000 rpm, using a homomixer. The emulsion wasused to carry out the subsequent steps, including a curing step and aneutralizing step, as in Example 1, so as to obtain amino resincrosslinked particles in the form of a white powder. The amino resincrosslinked particles had an average particle size 2.8 μm and a standarddeviation 1.1 μm. The same measurement and confirmation were carried outas in Example 1. The main reaction conditions and results are summarizedin Table 1.

Example 3

A heating step was carried out in an atmosphere of nitrogen (oxygenconcentration of 3 percent by volume). That is, amino resin crosslinkedparticles of a white powdery form was obtained as in Example 1 with theadditional heating step. The product amino resin crosslinked particleshad a Hunter whiteness of 93 percent.

Comparative Example 1

A reaction step and an emulsifying step were carried out as inExample 1. The reaction was allowed until the viscosity of the reactionsolution became 6.0×10⁻² Pa.s (60 cP), so as to obtain an emulsion thatcontained an amino resin precursor in a concentration of 52.5 percent byweight. The amino resin precursor in the emulsion had an averageparticle size 5.6 μm and a standard deviation 1.25 μm. The emulsion wascooled to 30° C. and a water dispersion of silica was added thereto asin Example 1. Then, a curing step was carried out as in Example 1,except that 160 g of DBS (dodecylbenzenesulfonic acid) was used.Thereafter, amino resin particles were separated from the suspension byfiltration, without carrying out the neutralizing step. The amino resinparticles thus separated were then heated for 5 hours at 100° C. andgently crushed with a pestle in a mortar. The result was amino resincrosslinked particles of a white powdery form. That is, in thisComparative Example 1, the neutralizing step and the heat treatment stepof a relatively high temperature (130° C. to 230° C.) were not carriedout. The amino resin crosslinked particles had an average particle size5.6 μm and a standard deviation 1.27 μm. The same measurement andconfirmation were carried out as in Example 1. The main reactionconditions and results are summarized in Table 1. In addition, theresult of solid-state ¹³C-NMR analysis on the product amino resincrosslinked particles is shown in FIG. 2.

Comparative Example 2

A reaction step and an emulsifying step were carried out as in Example1, so as to obtain an emulsion that contained an amino resin precursorin a concentration of 52.5 percent by weight. The amino resin precursorin the emulsion had an average particle size 2.4 μm and a standarddeviation 0.7 μm. The emulsion was cooled to 30° C. and a waterdispersion of silica was added thereto as in Example 1. Then, a curingstep was carried out as in Example 1. Thereafter, amino resin particleswere separated from the suspension by filtration, without carrying outthe neutralizing step. The amino resin particles thus separated werethen subjected to subsequent steps, including the heating step, as inExample 1. The result was amino resin crosslinked particles of aslightly yellowish white powdery form. That is, in this ComparativeExample 2, without the neutralizing step, the whiteness slightly droppedto 80 percent by the heating step. The amino resin crosslinked particleshad an average particle size 2.7 μm and a standard deviation 0.8 μm. Thesame measurement and confirmation were carried out as in Example 1. Themain reaction conditions and results are summarized in Table 1. TABLE 1COMPARATIVE COMPARATIVE EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 1 EXAMPLE2 MONOMER BG BG BG BG BG RESIN TEMPERATURE (° C.) 95 96 95 95 95VISCOSITY OF REACTION 4.5 5.5 4.5 6.0 4.5 SOLUTION AFTER REACTION (×10⁻²Pa · s) PVA (g/BG 100 g) 4 4 4 4 4 TEMPERATURE OF EMULSIFICATION (° C.)77 77 77 77 77 CONCENTRATION OF 52.5 52.5 52.5 52.5 52.5 EMULSIFICATION(WT %) AVERAGE PARTICLE SIZE AFTER 2.4 2.6 2.4 5.6 2.4 EMULSIFICATIONd50 (μm) STANDARD DEVIATION OF 0.7 1.01 0.7 1.25 0.7 PARTICLES AFTEREMULSIFICATION (μm) CURING CATALYST SULFURIC SULFURIC SULFURIC DBSSULFURIC ACID ACID ACID ACID AMOUNT OF CATALYST ADDED (WT %) 1.3 1.3 1.35.3 1.3 pH OF SUSPENSION AFTER NEUTRALIZATION 7.5 6.0 7.5 3.0 3.1HEATING TEMPERATURE (° C.) 150 150 150 100 150 AVERAGE PARTICLE SIZE 2.72.8 2.7 5.6 2.7 AFTER PULVERIZATION d50 (μm) STANDARD DEVIATION OF 0.81.1 0.8 1.27 0.8 PARTICLES AFTER PULVERIZATION (μm) NMR AREA RATIOC(II)/C(I) 3.86 3.60 3.86 0.67 3.83 C(II)/C(III) 0.13 0.12 0.13 0.060.12 C(II)/C(IV) 0.25 0.24 0.25 0.13 0.25 HUNTER WHITENESS (%) 92 91 9384 80 HUNTER WHITENESS AFTER 88 87 90 65 64 HEAT RESISTANCE TEST (%)SOLVENT RESISTANCE ∘ ∘ ∘ x ∘ LIGHT RESISTANCE ∘ ∘ ∘ x ∘ AMOUNT OF HCHOGENERATED (ppm) NOT NOT NOT 15000 NOT DETECTED DETECTED DETECTEDDETECTED ODOR VERY WEAK VERY WEAK VERY WEAK STRONG VERY WEAK

Note that, in Table 1, “BG” and “PVA” indicate benzoguanamine andpolyvinyl alcohol, respectively. The unit of PVA is gram per 100 g ofbenzoguanamine.

As is clear from the results in Table 1, by the neutralizing step forneutralizing the suspension that contains amino resin particles and bythe subsequent heating step for heating the amino resin crosslinkedparticles in a temperature range of 130° C. to 230° C., it is possibleto obtain amino resin crosslinked particles that have a Hunter whitenessof not more than 85 percent and that generate formaldehyde in an amountof not more than 1000 ppm in a pyrolysis test. The amino resincrosslinked particles that were produced without the neutralizing stephave a Hunter whiteness below 85 percent, i.e., the amino resincrosslinked particles discolored. Table 1 also shows that the aminoresin crosslinked particles that were heated at a temperature below 130°C. generate formaldehyde in a pyrolysis test and have poor solventresistance.

Note that, the amino resin crosslinked particles of Examples 1 and 2were additionally heated at 150° C. to observe any change of color overtime. After several hours, essentially no change of color was observedon the amino resin crosslinked particles.

(Colored Amino Resin Crosslinked Particles)

The following Examples 4 through 6, and Comparative Examples 3 and 4correspond to the Second Embodiment of the present invention.

Example 4

A 10 L reactor equipped with a stirrer, a reflux condenser, athermometer, and a vibration viscometer (product of MIVI ITS JAPAN;Model No. MIVI 6001) was charged with 3000 g (16 moles) ofbenzoguanamine as an amino compound, 2600 g of 37 percent by weightformalin (32 moles of formaldehyde), and 10 g of an aqueous solution of10 percent by weight sodium carbonate (0.01 mole of sodium carbonate).The temperature of the reaction mixture was raised with stirring to 95°C. to start a reaction.

The reaction was ended by cooling the reaction solution when theviscosity of the reaction mixture became 4.0×10⁻² Pa.s (40 cP).Separately, a dispersion of oil-soluble dye was prepared by adding andthoroughly dispersing 50 g of an oil-soluble dye (Fluorescent Red 632,the product of Arimoto Chemical Co., Ltd.) in an aqueous solutiondissolving 0.5 g of a dispersion auxiliary agent (product name EMULGEN920, provided by Kao Corporation) in 70 g of pure water. The dispersionso prepared was added to the reaction solution and the mixture wasstirred. The result was a colored reaction solution that contained anamino resin precursor, which is the initial condensate of benzoguanamineand formaldehyde.

Then, an aqueous solution dissolving 100 g of polyvinyl alcohol (productname PVA 205, provided by Kuraray Co., Ltd.) as an emulsion in 5150 g ofwater was charged in a 20 L reactor equipped with a reflux condenser, ahomomixer (stirrer, provided by Tokushu Kika Kogyo Co., Ltd.), and athermometer, etc. The temperature of the mixture was raised to 75° C.with stirring. After adding the reaction solution to the reactor, thetemperature of the reaction mixture was raised to 77° C. and the contentwas vigorously stirred at 7000 rpm at a maintained temperature of 77°C., so as to emulsify the amino resin precursor and obtain a pinkemulsion that contained the amino resin precursor in a concentration of38.3 percent by weight. A measurement of the emulsion with themultisizer (Coulter Multisizer II, the product of Coulter Inc.) showedthat the amino resin precursor in the emulsion had an average particlesize (d50) of 3.5 μm and a standard deviation of 0.62 μm. To the reactorwas then added, as a water dispersion of silica provided as an inorganiccompound, 3000 g of AEROSIL 200 (provided by Japan Aerosil) with a solidconcentration of 10 percent by weight. Thereafter, the content wasstirred with the homomixer for 5 minutes at 4000 rpm at a maintainedtemperature of 77° C. The resulting emulsion was cooled to 30° C.

Then an aqueous solution dissolving 42 g of concentrated sulfuric acidas a catalyst in 1200 g of pure water was added to the emulsion (at acontent temperature 30° C.), and the temperature of the reaction mixturewas raised with stirring to 90° C. at 10° C./hr. The reaction mixturewas maintained at 90° C. for an hour to condense and cure the aminoresin precursor (curing step). Thus, the total reaction time was 7hours.

After the curing step, the suspension containing amino resin particleswas cooled to 30° C., and the pH of the suspension was adjusted to 7.1,using a 5 percent by weight sodium hydroxide aqueous solution(neutralizing step). After the neutralizing step, the colored aminoresin particles according to the present invention were separated fromthe suspension by filtration. The amino resin particles thus separatedwas heated for 5 hours at 150° C. (heating step) and gently crushed witha pestle in a mortar. The result was a pink powder of amino resincrosslinked particles.

Measurements of the amino resin crosslinked particles with themultisizer showed that the amino resin crosslinked particles had anaverage particle size (d50) of 3.7 μm and a standard deviation of 0.99μm. The amino resin crosslinked particles were then placed for asolid-state ¹³C-NMR analysis, so as to measure heat discoloring, solventresistance, light resistance, and an amount of HCHO generated in apyrolysis test. In addition, odor was confirmed. The main reactionconditions and results are summarized in Table 2. The results ofsolid-state ¹³C-NMR analysis on the amino resin crosslinked particlesare shown in FIG. 3.

Example 5

A dispersion of oil-soluble dye was prepared by adding and thoroughlydispersing 50 g of an oil-soluble dye (Fluorescent Yellow 600, theproduct of Arimoto Chemical Co., Ltd.) in an aqueous solution dissolving0.5 g of a dispersion auxiliary agent (product name EMULGEN 920,provided by Kao Corporation) in 100 g of pure water. Then, a reactionstep, a coloring step, and an emulsifying step were carried out as inExample 4, except that the dispersion was added to the reaction solutionin the coloring step, so as to obtain a yellow emulsion that containedan amino resin precursor in a concentration of 38. 3 percent by weight.The amino resin precursor in the emulsion had an average particle size(d50) of 4.0 μm and a standard deviation of 1.19 μm. The emulsion wascooled to 30° C. To the reactor was then added, as a water dispersion ofsilica provided as an inorganic compound, 3000 g of AEROSIL 200(provided by Japan Aerosil) with a solid concentration of 10 percent byweight. Thereafter, the content was stirred with the homomixer for 5minutes at 4000 rpm. The emulsion was used in the subsequent steps,including the curing step and the neutralizing step, as in Example 4.Amino resin particles were then separated from the reaction solution byfiltration. Thereafter, a heating step (heat treatment) was carried outfor 5 hours at 150° C. as in Example 4. The result was amino resincrosslinked particles of a yellow powdery form.

The amino resin crosslinked particles had an average particle size 4.1μm and a standard deviation 1.30 μm. The same measurement andconfirmation were carried out as in Example 4. The main reactionconditions and results are summarized in Table 2.

Example 6

Amino resin crosslinked particles of a yellow powdery form were obtainedby the method of Example 5, except that the heating step was carried outin an atmosphere of nitrogen (oxygen concentration of 7 percent byvolume). The product amino resin crosslinked particles had a colordifference (ΔE*_(ab)) of 6.0 in a heat resistance test. The value of Δbwas 4.5. The main reaction conditions and results are shown in FIG. 2.

Comparative Example 3

A reaction step and an emulsifying step were carried out as in Example4, so as to obtain a pink emulsion that contained an amino resinprecursor in a concentration of 38.3 percent by weight. The amino resinprecursor in the emulsion had an average particle size 3.6 μm and astandard deviation 1.02 μm. After adding a water dispersion of silica asin Example 4, the emulsion was cooled to 30° C. Then, a curing step wascarried out as in Example 4. Amino resin particles were then separatedfrom the suspension by filtration, without carrying out the neutralizingstep. The amino resin particles so separated were heated for 5 hours at150° C. and gently crushed with a pestle in a mortar. As a result,comparative amino resin crosslinked particles of a pink powdery formwere obtained. That is, in this Comparative Example 3, the neutralizingstep was not carried out.

The comparative amino resin crosslinked particles had an averageparticle size 3.8 μm and a standard deviation 1.40 μm. The samemeasurement and confirmation were carried out as in Example 4. The mainreaction conditions and results are summarized in Table 2.

Comparative Example 4

A reaction step and an emulsifying step were carried out as in Example5, so as to obtain a yellow emulsion that contained an amino resinprecursor in a concentration of 38.3 percent by weight. The amino resinprecursor in the emulsion had an average particle size 3.9 μm and astandard deviation 1.15 μm. After cooling the emulsion to 30° C., awater dispersion of silica was added as in Example 5.

The emulsion was used to carry out a curing step as in Example 4, exceptthat 170 g of DBS (dodecylbenzenesulfonic acid) was used. Then, aminoresin particles were separated from the suspension by filtration,without carrying out the neutralizing step. The amino resin particles soseparated were then heated for 5 hours at 150° C. and gently crushedwith a pestle in a mortar. As a result, comparative amino resincrosslinked particles of a yellow powdery form were obtained. That is,in this Comparative Example 4, the neutralizing step and the heattreatment step of a relatively high temperature (130° C. to 230° C.)were not carried out.

The comparative amino resin crosslinked particles had an averageparticle size (d50) 4.0 μm and a standard deviation 1.35 μm, as given bya multisizer. The same measurement and confirmation were carried out asin Example 4. The main reaction conditions and results are summarized inTable 2. TABLE 2 COMPARATIVE COMPARATIVE EXAMPLE 4 EXAMPLE 5 EXAMPLE 6EXAMPLE 3 EXAMPLE 4 PINK YELLOW YELLOW PINK YELLOW MONOMER BG BG BG BGBG RESIN TEMPERATURE (° C.) 95 96 95 95 95 VISCOSITY OP REACTION 4.0 4.04.0 4.0 4.0 SOLUTION AFTER REACTION (×10⁻² Pa · s) PVA (g/BG 100 g) 3 33 3 3 TEMPERATURE OF EMULSIFICATION (° C.) 75 75 75 75 75 CONCENTRATIONOF 38 38 38 38 38 EMULSIFICATION (WT %) AVERAGE PARTICLE SIZE AFTER 3.54.0 3.5 3.6 3.9 EMULSIFICATION d50 (μm) STANDARD DEVIATION OF 0.62 1.190.62 1.02 1.15 PARTICLES AFTER EMULSIFICATION (μm) CURING CATALYSTSULFURIC SULFURIC SULFURIC SULFURIC DBS ACID ACID ACID ACID AMOUNT OFCATALYST ADDED (WT %) 1.3 1.3 1.3 1.3 5.7 pH OK SUSPENSION AFTERNEUTRALIZATION 7.1 7.1 7.1 3.1 3.0 HEATING TEMPERATURE (° C.) 150 150150 150 100 DRYING TIME (h) 5 5 5 5 5 AVERAGE PARTICLE SIZE 3.7 4.1 3.73.8 4.0 AFTER PULVERIZATION (μm) STANDARD DEVIATION OF 0.99 1.30 0.991.30 1.35 PARTICLES AFTER PULVERIZATION (μm) NMR AREA RATIO C(II)/C(I)3.76 3.55 3.76 3.75 1.02 C(II)/C(III) 0.12 0.13 0.12 0.12 0.06C(II)/C(IV) 0.26 0.24 0.26 0.25 0.13 COLOR DIFFERENCE VALUE (ΔE*_(ab))3.1 7.7 6.0 17.5 20.1 Δb* VALUE 1.6 6.0 4.5 11.5 14.0 SOLVENT RESISTANCE∘ ∘ ∘ ∘ x LIGHT RESISTANCE ∘ ∘ ∘ ∘ x AMOUNT OF HCHO GENERATED (ppm) NOTNOT NOT NOT 14000 DETECTED DETECTED DETECTED DETECTED ODOR VERY WEAKVERY WEAK VERY WEAK VERY WEAK STRONG

Note that, in Table 2, “BG” and “PVA” indicate benzoguanamine andpolyvinyl alcohol, respectively. The unit of PVA is gram per 100 g ofbenzoguanamine.

As is clear from the results in Table 2, by the neutralizing step forneutralizing the suspension that contains amino resin particles and bythe subsequent heating step for heating the amino resin crosslinkedparticles in a temperature range of 130° C. to 230° C., it is possibleto obtain amino resin crosslinked particles that have a color differenceof not more than 15 in a heat discoloring test and that generateformaldehyde in an amount of not more than 1000 ppm in a pyrolysis test.The amino resin crosslinked particles that were produced without theneutralizing step have a color difference greater than 15 in a heatdiscoloring test, i.e., the amino resin crosslinked particlesdiscolored. Table 2 also shows that the amino resin crosslinkedparticles that were heated at a temperature below 130° C. generateformaldehyde in a pyrolysis test and have poor solvent resistance.

The following Examples 7 through 9 are based on a process that comprisesthe steps of (1) adding a catalyst to an emulsion of an amino resinprecursor, which is the product of a reaction of an amino compound withformaldehyde, in an aqueous solution of a surfactant, so as to cure theamino resin precursor and obtain amino resin particles; and (2) heatingthe amino resin particles in an atmosphere of inert gas that containsoxygen in a concentration of not more than 10 percent by volume and in atemperature range of 130° C. to 230° C. That is, the following Examples7 through 9 do not limit the present invention in any ways. In otherwords, the following Examples 7 through 9 and Comparative Example 5correspond to the Third Embodiment of the present invention.

Example 7

A four-neck flask equipped with a stirrer, a reflux condenser, andthermometer was charged with 150 parts of melamine, 290 parts by weightof formalin in a concentration of 37 percent by weight, and 1.5 parts ofammonium water in a concentration of 28 percent by weight, so as toprepare a reaction mixture. The pH of the system was adjusted to 8.0.The temperature of the reaction mixture was raised to 70° C. withstirring and a reaction was allowed for 30 minutes at this temperature,so as to obtain an initial condensate with miscibility withwater∞percent.

Separately, 12 parts by weight of an anionic surfactant (NEOPELEX No. 6Fpowder (the product of Kao Corporation, sodium dodecylbenzenesulfonate)was dissolved in 2240 parts by weight of water. The aqueous solution ofthe surfactant was stirred at an increased temperature of 90° C. Then,the initial condensate was placed in the aqueous solution of thesurfactant being stirred. Additionally, 75 parts by weight of an aqueoussolution of 10 percent by weight dodecylbenzenesulfonic acid was added.Then, the temperature of the reaction mixture was gradually raised to90° C. The reaction mixture was maintained for 2 hours at thistemperature so as to obtain a suspension of cured resin by condensationand curing. Observing the suspension under a light microscope (800 timesmagnification) showed that the particles were spheres with a particlesize of about 0.5 μm. Active Brownian motion of the particles was alsoobserved.

The suspension was cooled to 30° C. and the pH thereof was adjusted to7.0 using a 5 percent by weight sodium hydroxide aqueous solution. Afteradding 200 parts by weight of a 1 part by weight aluminum sulfateaqueous solution, the emulsion was placed for suction filtration andsolid-liquid separation. The cured resin so separated was then dried byheating for 2 hours using a hot-air drier at 160° C. (heat treatmentstep) in an atmosphere of nitrogen (oxygen concentration of 0.5 percentby volume), so as to obtain an agglomerate of 183 parts by weight curedresin spherical particles. The agglomerate was then crushed using a ballmill to obtain cured resin spherical particles of a white color (aminoresin crosslinked particles). Observing the cured resin sphericalparticles under a scanning electron microscope showed that the particleswere uniform spheres with an average particle size of 0.5 μm. The curedresin spherical particles had a Hunter whiteness of 94 percent. Theresults are shown in Table 3.

Example 8

A four-neck flask equipped with a stirrer, a reflux condenser, andthermometer was charged with 75 parts of melamine, 75 parts by weight ofbenzoguanamine, 290 parts by weight of formalin in a concentration of 37percent by weight, and 1.5 parts of a sodium carbonate aqueous solutionin a concentration of 10 percent by weight, so as to prepare a reactionmixture. The pH of the system was adjusted to 8.0. The temperature ofthe reaction mixture was raised to 85° C. with stirring and a reactionwas allowed for 1.5 hours at this temperature, so as to obtain aninitial condensate with miscibility with water 200 percent.

Separately, 7.5 parts by weight of a nonionic surfactant (EMULGEN 430(the product of Kao Corporation, polyoxyethyleneoleilether) wasdissolved in 2455 parts by weight of water. The aqueous solution of thesurfactant was stirred at an increased temperature of 50° C. Then, theinitial condensate was placed in the aqueous solution of the surfactantbeing stirred, so as to obtain an emulsion of the initial condensate. Tothe emulsion was added 90 parts by weight of an aqueous solution of 5percent by weight dodecylbenzeneslfonic acid. The reaction mixture wasthen maintained for 3 hours n a temperature range of 50° C. to 60° C.,so as to obtain an emulsion of cured resin. The emulsion was rapidlycooled in 3000 parts by weight of cold water. Observing the emulsion socooled under a light microscope (600 times magnification) showed thatthe particles were highly uniform spheres with an average particle sizeof about 8 μm.

Then, a paste obtained by precipitation separation of the cured resinfrom the emulsion was dissolved in 2000 parts by weight of water,together with EMULGEN 430 (7.5 parts by weight) and 4.5 parts by weightof dodecylbenzenesulfonic acid. The cured resin was then dispersed inthe aqueous solution using a ultrasonic dispersing device. Thetemperature of the cured resin so dispersed was then gradually increasedto 90° C. with stirring and maintained at this temperature for an hour,so as to completely cure the cured resin. As a result, a suspension ofsufficiently cured resin was obtained. The cured resin was separatedfrom the suspension by centrifugal separation and dried by heating for 4hours with a hot-air drier at 150° C. (heat treatment step) in anatmosphere of nitrogen (oxygen concentration of 2 percent by volume), soas to obtain an agglomerate of 120 parts by weight cured resin sphericalparticles. The agglomerate was then crushed with a ball mill to obtainamino resin crosslinked particles of a white color (cured resinspherical particles). Measuring the amino resin crosslinked particleswith a particle distribution measuring device (Coulter Counter ModelTA-II, C-1000, the product of Coulter Electronics, Inc.) showed that theamino resin crosslinked particles had a notably narrow particledistribution with an average particle size 8.0 μm and a standarddeviation 0.5 μm. Observing the amino resin crosslinked particles undera scanning electron microscope showed that the particles were sphereswith an average particle size of about 8 μm. Further, the amino resincrosslinked particles had a Hunter whiteness of 90 percent. The resultsare shown in Table 3.

Example 9

White amino resin crosslinked particles were obtained by the method ofExample 1, except that the neutralizing step, which was carried out inExample 7, was not carried out in this Example. That is, in this Example9, a heating step was carried out in an atmosphere of nitrogen. Theamino resin crosslinked particles had a Hunter whiteness of 89 percent.The results are shown in Table 3.

Example 10

White powdery amino resin crosslinked particles were obtained by themethod of Example 1, except that the neutralizing step using 5 percentby weight sodium hydroxide after the curing step was not carried out,and the heating step was carried out in an atmosphere of nitrogen(oxygen concentration of 3 percent by volume). The amino resincrosslinked particles had a Hunter whiteness of 89 percent. The resultsare shown in Table 3.

Comparative Example 5

White amino resin crosslinked particles were obtained by the method ofExample 7, except that the heating step (heat treatment step) wascarried out in an atmosphere of air, instead of nitrogen, and theneutralizing step using an aqueous solution of 5 percent by weightsodium hydroxide was not carried out. That is, in this ComparativeExample, the neutralizing step and the heating step (heat treatmentstep) in an atmosphere of inert gas were not carried out. The aminoresin crosslinked particles had a Hunter whiteness of 82 percent. Theresults are shown in Table 3. TABLE 3 COMPARATIVE EXAMPLE 7 EXAMPLE 8EXAMPLE 9 EXAMPLE 10 EXAMPLE 5 NMR AREA RATIO C(II)/C(I) 3.95 3.80 3.953.86 3.95 C(II)/C(IV) 0.32 0.35 0.32 0.25 0.32 HUNTER WHITENESS (%) 9490 89 89 82 HUNTER WHITENESS AFTER 92 86 86 86 65 HEAT RESISTANCE TEST(%) SOLVENT RESISTANCE ∘ ∘ ∘ ∘ ∘ LIGHT RESISTANCE ∘ ∘ ∘ ∘ ∘ AMOUNT OFHCHO NOT NOT NOT NOT NOT GENERATED (ppm) DETECTED DETECTED DETECTEDDETECTED DETECTED ODOR VERY WEAK VERY WEAK VERY WEAK VERY WEAK VERY WEAK

As is clear from the results shown in Table 3, by carrying out theheating step in an atmosphere of inert gas, discoloring of the aminoresin crosslinked particles can be suppressed. Further, by carrying outthe neutralizing step in combination with the heating step (heattreatment step) in an atmosphere of inert gas, discoloring of the aminoresin crosslinked particles can be suppressed more effectively.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1-12. (canceled)
 13. A producing process of amino resin crosslinkedparticles, comprising the steps of: (1) adding a catalyst to an emulsionof an amino resin precursor that is obtained by a reaction of an aminocompound with formaldehyde, the emulsion being a mixture of a reactionsolution containing the amino resin precursor with an aqueous solutionof an emulsifier and/or a surfactant, so as to cure the amino resinprecursor in an emulsion state to prepare a suspension that containsamino resin particles; (2) neutralizing a suspension that contains theamino resin particles obtained in said step (1); and (3) separating theamino resin particles from the suspension after said step (2) and thenheating the amino resin particles in a temperature range of 130° C. to230° C., the suspension containing the amino resin particles beingadjusted in said step (2) from a pH of 1.5 to 3 to a pH of not less than5.
 14. The process as set forth in claim 13, wherein said step (3) iscarried out in an atmosphere of inert gas that contains oxygen in aconcentration of not more than 10 percent by volume.
 15. The process asset forth in claim 13, wherein the amino resin precursor is obtained asan initial condensate in a reaction solution that results from thereaction of the amino compound with the formaldehyde, the reactionsolution containing the amino resin precursor having a viscosity of2×10⁻² Pa.s to 5.5×10⁻² Pa.s (20 cP to 55 cP) in a temperature range of95° C. to 98° C.
 16. The process as set forth in claim 13, wherein saidstep (1) comprises a step of coloring the amino resin precursor with adye and/or a pigment.
 17. The process as set forth in claim 16, whereinthe dye is a fluorescent dye and the pigment is a fluorescent pigment.18. A producing process of amino resin crosslinked particles, comprisingthe steps of: (1) adding a catalyst to an emulsion of an amino resinprecursor that is obtained by a reaction of an amino compound withformaldehyde, the emulsion being a mixture of a reaction solutioncontaining the amino resin precursor with an aqueous solution of anemulsifier and/or a surfactant, so as to cure the amino resin precursorin an emulsion state to prepare a suspension that contains amino resinparticles; and (2) heating the amino resin particles in an atmosphere ofinert gas that contains oxygen in a concentration of not more than 10percent by volume and in a temperature range of 130° C. to 230° C. 19.The process as set forth in claim 18, wherein said step (1) comprises astep of coloring the amino resin precursor with a dye and/or a pigment.20. The process as set forth in claim 19, wherein the dye is afluorescent dye and the pigment is a fluorescent pigment.